I disagree. When we look back at the dissapointing nominations- O'Connor, we have to keep in mind there was a Democratic Senate. So while we may have had a Republican President, it was not enough to seal the deal. We had to compromise- or we wouldn't get anyone through.
That might excuse the Presidents in some of those cases (actually, Reagan had a GOP Senate, except at the very end), but it doesn't excuse the fact that at the time, most conservatives thought that these nominees were conservatives.
The primary examples, of course, being Kennedy for Bork (what a missed opportunity!) and Souter (IIRC pushed by Warren Rudman, backed by Sununu, and nominated to avoid a confrontation).
BTW, I've read (I don't recall myself) that Kennedy started off pretty conservative -- don't know about O'Connor. But Mark Levin once said that the justices from out of town who get into the DC social scene are the ones who drift left.
Stevens, of course, was appointed by Ford, who was not a principled conservative and had no wish to pick a fight with Dems, given the timing.