I've decided to use the Roberts confirmation battle as a litmus test. Any liberal who supports him is sensible. The ones who oppose him are not. It's that simple.
Edwin Chemerinsky, a first-class scholar on the issue of religion and state, argued that liberals must oppose Roberts for the same reason they opposed Bork: because he is against Roe v. Wade, argued against the right of privacy, and supports a greater role for religion in the public sphere. He also believes his short tenure on the Court of Appeals proves Roberts is an extremist conservative.
Like many others, Chemerinsky incorrectly assumes that work offered as part of employment is indicative of personal views. Lawyers are often required to make arguments on behalf of their client. In this case, that was the Reagan administration, which opposed abortion rights and denied the existence of a constitutional right of privacy. So of course Roberts would oppose it. That was his job.
Chemerinsky also claims that his memos show he's an archconservative. I've written memos in school arguing against my personal preference. Memos are written for bosses, who have a certain position in mind.
Roberts tenure on the Court of Appeals was too short to be reflective of his possible jurisprudence on the Supreme Court.
Chemerinsky also ignores the distinction between Bork's clear-cut jurisprudence, which is strict textualism-originalism, and Robert's more Rehnquist-like conservatism. I've seen nothing that shows me that he's an originalist, which is precisely why some conservatives oppose him.
One last point: George W. Bush is a conservative. He's going to pick a conservative justice. There are many worse candidates (from the liberal point of view) out there.
(Hat Tip: Volokh Conspiracy)