There are three things you can say for sure about Sarah Palin:
1) Her selection as John McCain's running mate was a huge surprise
2) She has less experience than any candidate on a major party ticket than anyone I can think of...maybe less than anyone in the history of the country. (Certainly the Democrats will try to portray it that way.)
3) The choice has given the McCain campaign a short-term boost in terms of attention paid to it and money contributed to it, and put a pitchfork into Obama's recent ownership of TV news coverage.

Let's look briefly at Governor Palin's weaknesses and strengths:

Weaknesses

By far, Palin's biggest weakness is her lack of experience. Although some may try to paint this with the "political outsider" brush, there is no way to make Palin's thin political resume anything other than a substantial net negative, particularly running with a candidate who would be the oldest president ever at the time of taking office. Not only does her lack of experience open Palin to criticism, but it makes it very difficult for the McCain campaign to go after Obama's dismal and short record.

That said, it bears mentioning that two of the best political minds I know believe that the experience argument "won't work where a woman is concerned", basically because there are so few women with strong resumes who could be selected that relative inexperience is likely to be the case at this point in time for a woman. This choice is a huge step forward for women overall, and therefore risky to criticize, they believe.

Her second biggest weakness should barely even be mentioned in the same article as her experience because it is so much less significant. It revolves around an inquiry into Palin's firing of a public employee who had himself refused to fire Palin's ex-brother-in-law who apparently had a nasty divorce (his fourth) from Palin's sister. The brother-in-law also appears to have had a far-from-stellar record as a police officer and was accused by the Palin's of alcohol abuse and of threating Palin's sister, father, and others. My guess is that McCain asked Palin about this directly, i.e. whether there was a political motivation behind the firing, in addition to whether she had any other skeletons that would embarrass the campaign, and he must have been satisfied with the answer.

A third indirect weakness is that her selection lets people try to characterize John McCain as a gambler...something which can't be helpful. This is somewhat offset by the fact that people will be hesitant to attack a successful female politician.

Something which is both a weakness and a strength is that she's the mother of five children, the youngest of which has Down Syndrome. It will make her more appealing to women voters who aren't committed liberals. But Obama supporters are also asking publicly whether she can be a good parent and a good VP at the same time. And while that question is something that would never be of a man in the same situation, I can imagine that it would stick in some people's minds...sort of the way that the McCain ads comparing Obama to Paris Hilton were possibly unfair but certainly effective.

Governor Palin's strengths are more in number but individually (and maybe in the aggregate) less important than her major weakness.

Strengths

Roughly in order of importance...

She has a solid reputation for being for good government, for fighting corruption and waste, particularly within her own party, so she adds to the "maverick" reputation of the ticket. I (and I presume many others) were impressed when she endorsed a primary challenger to Congressman Don Young. (As I write this, several days after the Alaska primary, the winner of that race has still not been determined.)

[She should continually contrast her record of fighting waste and corruption, even if that record is short, with Biden's 30 years of big spending in the Senate and with Obama's consistent support for the corrupt Democratic Party Machine and wasteful spending in Illinois.]

The experience she does have is executive, rather than legislative, and I don't think any of the other three people on the tickets have executive experience. This can somewhat neutralize the experience argument, especially versus Obama.

She's a woman, so she might appeal to the segment of disaffected Hillary voters who care more about whether the candidate is a woman than if she's a liberal. I believe that's a small fraction of Hillary supporters (i.e. most of them would only support another liberal), but in elections this close even a small fraction could make a big difference.

Despite her short tenure so far as governor, she is already more credible than any of the other three candidates on the major party tickets on the issue of energy.

Her social conservative credentials will help McCain with the GOP base, but she doesn't seem like such a fire-breathing social conservative that moderates would rule out voting for McCain because of her.

She's not bad looking.

There's a big difference between an inexperienced candidate for VP and an inexperienced candidate for president. However, John McCain's age makes this distinction less than it otherwise might be.

Other minor issues

Although her husband's primary job is as a commercial fisherman, be certain that the liberals will play up his other job...working for a major oil company.

She's very hard to pigeon-hole: She and her husband are both union members. She's a hunter, known for shooting and eating moose. She seems both quite intelligent and down to earth, but knows little about foreign policy. You can bet she's being coached on that and other policy areas as many hours a day as she can tolerate.

Putting it all together

McCain's choice of Sarah Palin is a very big gamble. It was probably a good tactical move, in the sense of bringing some excitement to the GOP side of the campaign, rather than having the entire thing being about Obama, despite Obama's protestations that it's not about him. It helped raise some money. It crushed Obama's media cycle for the long holiday weekend like a boot grinding out a lit cigarette.

If the election were tomorrow, I'd say it was probably a brilliant choice.

But the election isn't tomorrow, and I think the markets have it right when they show basically no change in McCain's chances of winning between 24 hours before the Palin announcement and 24 hours afterward.

Sarah Palin comes across well on TV when talking about things she knows about, such as Alaska's energy production. However, it remains to be seen whether she can keep up with the well coiffed, highly polished, and very experienced (especially on foreign policy and certain law enforcement issues) Joe Biden. One has to also remember that Democrats, including Biden, seem perfectly happy to plagiarize or lie even in front of a large audience, which can make it more difficult for a debater who is speaking only what she believes to be accurate information or her own opinion (rather than someone else's masquerading as her own.)

Governor Palin will have to be nearly flawless during the last two months of the campaign, and particularly in her debate(s), in order for her selection to be a long-run net positive, despite the obvious short-term benefits to McCain. It is not easy for even the best politicians to be that good for that long. While I have no reason to think that Palin is one of the best, the fact that she has high popularity ratings in Alaska and is, at first glance, quite appealing on many levels, certainly means that she could be a long-term positive surprise as well as a short-term one. Maybe she can pull it off, but it would be like a baseball player hitting safely for 20 games in a row. It can be done, and even by players that you might otherwise never think of as particularly special, but it sure isn't easy.

Despite the risks of choosing her, I believe I understand why McCain did it. Romney had some clear positives, the greatest of which is his obvious superior knowledge and talent in the areas of business and economics...an area where McCain, Obama, and Biden are all weak. But he would not have been an exciting choice, he carried some baggage (his religion, his socialized medicine plan in Massachusetts, and his relatively recent convenient move to the right on social issues), and he probably wouldn't have made anything like the difference in short-term news coverage that Palin has. Also, there are consistent rumors that Romney and McCain just don't like each other very much. (For the record, the people I know who have met or worked with Romney all like him very much and the people I know who have met or worked with McCain think he has a big ego and generally don't like him on a personal level.)

So, after possibly too many words, my verdict on McCain's selection of Sarah Palin as running mate: Conceivably the best move he could have made, but primarily because most of his other possible choices were not obviously politically winning choices and because his campaign was suffocating. Despite all the understandable short-term excitement, it would not surprise me if Republican Party faithful and McCain supporters end up writing campaign post-mortems saying "If only he had picked Romney", particularly if the choice of Palin doesn't seem to bring in many women's votes. The good news is that Biden is probably not much better a choice than Palin, strategically speaking.

I hope that I'm wrong about Palin possibly being a better short-term decision than long-term (and I reiterate that smarter people than I think she's a "brilliant choice"). The more I hear from Obama, the clearer it becomes to me how dangerous a person he is and how, despite his flowing and occasionally patriotic rhetoric, his political and economic views are more anti-American and anti-capitalist than any major party candidate of my lifetime.