I love DePalma, and I think Dressed to Kill is actually one of his better works, but I've heard this same reaction from a number of friends. I guess if you're not already on his wavelength it could seem pretty ridiculous -- he's sort of a postmodern guy in the sense that there's a lot of b-movie satire/homage mixed in with the suspense. If the dialogue and direction seemed kind of stagey (I'm reading into your comments here) that might be partially why. And if that sort of thing doesn't float your boat, you probably won't like much of his other stuff.
I find the best parts of his movies are the major set-pieces -- that dream sequence you mentioned, the prom scene in Carrie, or Amy Irving's first big bloody vision in The Fury. Something I love about those scenes, but which I know puts a lot of people off, is his use of totally non-naturalistic techniques like rear projection, split screens, split focus, or even those Klieg lights in the background of the DTK dream sequence. He's always drawing your attention to the movie-ness of the experience.
Anyhow, if you want to keep trying, I'd highly recommend Carrie, followed by Blow-Out. Carlito's Way is a lot of fun (it features an incredible Sean Penn performance and Al Pacino is fairly restrained in it). More recently Mission to Mars had its moments.