I think "feel" is pretty significant. When I first got my SPDs, the first thing I noticed is that I was getting tired more quickly. Then I started looking at my bike computer and noticed I was maintaining higher speeds than normal. Its probably just psychological, but the change definitely increased the "weeeeeeeee!" factor.

My biggest problem with them though, is the whole changing shoes. I do a lot of riding to get somewhere rather than riding just to ride. Having to have special bike shoes is annoying.