What Retro Grouch says. I'll just add that once you've gone up a level, it's tough to go back down. As you ride more, you begin to notice the little things like crisper shifting, better finish, etc., that Retro Grouch refers to.

If you were buying a mountain-bike, I'd suggest aiming for a mix of LX and XT components. Those are were I believe you get solid quality and value for your dollar. I am not sure how to translate that advice to Shimano's road groupings.