I thought I would put this up here after all of the discussion about various types of suspension set-ups. others should feel free to add, as i am in no means an expert.

in my experience, both dual suspension and hardtails have their place. i've raced both, and have been quite successful on both of them. the main difference between the two is feel. when i was on my dual suspension, it felt sluggish going up hill. it didnt quite have the pop when i'd mash on the pedals. this was a >$2000 bike as well, so it wasn't crappy suspension. however, going downhill, it was much smoother as every little rock didn't throw the rear wheel off line. i couldn't tell if it was faster or not, but i do know that i had to pay less attention when descending as the bike could do most of the work. also, it was comfortable enough to actually be able to pedal through rock gardens and feel balanced.

i switched to a hardtail primarily because it was much quicker climbing, and i felt that the lighter weight bike would more than make up for the discomfort. after rides of 4 hours or so, my back starts to miss the suspension. the weight difference is something that can be felt while riding as well. additionally, the ridgid rear end allows me to know exactly where the rear tire is and what it's doing at all times. i like this quality in a bike. this means that i'm enabled to put the rear wheel right where i want it in technical sections, and i know how it will respond immdiately. in all, i think this is a faster bike than that dual suspension bike because of the weight and "ride feel".

those are my notes, feel free to discuss... ;)