After having seen Inception, I'm now qualified to at least comment on the idea that Paprika (my review) was a better treatment of the same idea, or at least a more fanciful one. This is a little like saying an omelet is a better hamburger, or something to that effect.
Heck, I myself was saying "Oh, it's Paprika" before the movie came out. Then I got into the theater, slid down into the seat, shut up, and actually watched it. After something like fifteen years of watching movies and reviewing them (albeit non-professionally), I've learned that it helps to push other things out of your mind when you're in the film, and then bring them back in when it's over.
When I walked out of Inception, I saw more differences than similarities. Both movies used the dreamspace as an arena, but to markedly different ends and with entirely different mechanics. That doesn't make one cleverer than the other, or a better use of the dreamscape. If the whole point of the dreamscape is that it's malleable and that it can be most anything you want, then both films work just fine in that respect. They use the dreamscape as raw material to serve their larger ends.
(Side note: the fact that the dreamscape in Inception is relatively grounded in reality I actually found refreshing, because it set up that many more ground rules, and made their work in the dream world that much more like work, with real consequences and boundaries to be observed.
(Side note 2: From what I've gathered, there are people who while watching some movies register a certain subconscious dislike for them ["subconscious security", anyone?], and then cherry-pick any negative criticism they can find to support their feelings. The same applies in reverse, of course, where people look for anything they can find to justify a movie they like. I'm of the feeling that if you like a movie, just like it because you like it, then the hell with any justification for it. If you hate it, same deal - just don't expect, in either case, other people to automatically follow.)
I enjoyed both films for what they were, just as I didn't think about the 1984 movie Dreamscape while watching either one — or, for that matter, when I was reading Yasutaka Tsutsui's novel, which apparently Wolfgang Petersen is interested in adapting himself into a live-action production. I wonder if that project's going to be scotched because people will think "Oh, it's Inception."
There's plenty of reasons to criticize either movie, but I'm not convinced this is one of them. It's a red herring at best.