President Obama was never going to appoint a Supreme Court justice in the Thomas-Alito mold, or even in the Kennedy-O'Connor style. Given that he's a liberal president, Elena Kagan is, on most counts, a laudable choice. She's got a pragmatic reputation, and has shown respect for conservative viewpoints. On substance, she's probably a loyal liberal, especially taking a leftist view of equal protection. But she seems more likely to make middle-of-the-court deals and less likely to hold out for extreme judgements or alienate average Americans with solicitude toward "protected" groups.
The one strike against Ms. Kagan is that she has never served as a judge at any level. Her legal expertise is beyond questioning, but jurists have been known to metamorphosize upon appointment to the high court (see: Souter, David) and those with short records are especially risky in that regard.
Republicans should give her a rigorous and respectful hearing, and make her go on record on the key procedural issues of the role of the high court. But using the filibuster or fulminating publicly about Kagan would merely show Obama that compromise and conciliation are worthless.