I'm making a rare reversion back into politics, because the Supreme Court is important.
Obviously, Sonia Sotomayor's ethnicity and her gender are hot topics. The way I see it, there are somewhere around 100 people in this country who are qualified for the Supreme Court at any given time. We may not agree with their decisions, but they have the training, background, and experience to be qualified. (I think that both of W's appointees, Roberts and Alito, fall into the 'qualified' category, although his first choice, Harriet Miers, definitely does not.)
So the President has to choose someone from a pool of people, all of whom have roughly the same set of qualifications. I think that it's worthwhile if the Supreme Court resembles the people of this country, and since Sotomayor comes from the set of people who are eligible due to their experience, she's a good choice.
Sonia Sotomayor is unquestionably qualified. None of the present justices ever worked as a state prosecutor, and only Alito, who worked as a federal prosecutor, has ANY criminal law experience. Hard to believe, but true. NONE of the present justices ever served as a trial judge (Souter did, but he's the one who's retiring). Since the justices regularly have to decide questions that are a trial judge's bread and butter (evidence, procedure, jury instructions, etc.), it would be worthwhile to have someone in the room who's actually had to make those kind of decisions.
So what kind of justice will she be? I don't have any idea. No one does. Souter was supposed to be a pocket conservative, and Scalia was seen as a moderate consensus builder. Who knows how someone will be in ten or twenty years? Not me.
One can demand that the President chooses someone who falls in the 'qualified' group, but outside that, you pretty much just have to hope they rule the way you want when the time comes.