Mark Tushnet, famous liberal constitutional scholar and opponent of judicial review, argues that Democratic Senators should vote against Roberts if they disagree with his vision of the Constitution. Moreover, since we know he had positions that are contrary to liberal jurisprudence (well, we at least know he argued for those positions), they should place the burden of persuasion on him to prove he's become more "centrist."
He also argues that the President and Senate are coequal parties in the confirmation process. I'm not knowledgeable enough to debate that issue, but, for the most part, it seems that historically the Senate has deferred to the President's nomination unless the candidate was grossly incompetent or corrupt. Until Bork that was the standard procedure. Since Roberts is neither, why vote against him?
Tushnet disagrees with a quid pro quo that requires Senators to allow the other parties' choice because otherwise the opposition party will make their confirmations difficult. The quid pro quo is a bad idea because the Democrats broke it with Bork. And the Republicans supported Ginsburg despite her "out of the mainstream" ideological views.
There's nothing inherently wrong with voting against a nominee for any reason. Kennedy could vote "NO" because he doesn't like Roberts' shirt. But politically that's a dumb move because one day the Democrats (if they get their act together) will be back in the White House, and they will want to get their nominee confirmed. It's not going to happen if the Democrats play this game.