I use Expose constantly! I have Hot Corners set up for it, and I also have buttons on my mouse (Logitech MX Revolution) set up for it. Depending on what I'm doing, one way of activating Expose is more useful than another. This is also in addition to using cmd-tab very frequently.

Some posters have said that Expose isn't useful if you're using multiple monitors, but I would have to respectfully disagree. I use Expose no matter if I'm on a multiple monitor MacPro setup, my 20' iMac, or my tiny-screened Dell Mini 10v hackintosh. Expose is, in my opinion, one of the greatest aspects of OS X. And when I see other Mac users not using Expose, it drives me nuts! It saves so much time, yet some people prefer to click on the Hide button in their windows. Why, oh God why?!?!?! hahaha

Spaces on the other hand, I have yet to really use. When I first got Snow Leopard, I used it a little bit, but mainly just because it was a feature I didn't have until then (I had Tiger before SL, skipped Leopard). I have yet to find a good use for Spaces, but I'm glad it's there, in case I ever find that I suddenly have a use for it.