I've had mixed results with older titles on Blu-ray, myself.
The Criterion recommendation is a good one, especially as one doesn't tend to pay as much of a Criterion tax on Blu-rays compared to DVDs. Still, I bought Last Year at Marienbad from Criterion and I'm not sure the higher definition helped. The images had a great deal of noise--perhaps it's possible that Blu-ray can show the grain of the film at that resolution? Maybe it was more noticeable because it was in black and white. I didn't have similar issues with Criterion's El Norte, which was in color.
I've generally had pretty good experience with the technicolor classics, like North by Northwest and Rio Bravo. I can't quantify it, but it seems like the color saturation is much more intense on Blu-ray. I've got Casablanca, The Searchers, and Gone With the Wind in my to-be-watched stack, I can report back on those when I get around to them.
I think the real litmus test would be something filmed in Panavision, like Lawrence of Arabia.
A classic whose quality really surprised me on Blu-ray was Clockwork Orange. 2001 looks decent as well. I keep hoping for Barry Lyndon to make an appearance in the format, because of the legendary technology Kubrick used.
I was amazed at how muddy and dated Total Recall looked on Blu-ray. I guess is shows that just because something is more recent, it doesn't mean that it will look great. I wonder if many of the big hits of the 90s and 80s may get lost in the shuffle over time with the better sort of home-viewing technology we have.