When I looked at buying my bike, I tried several twist shifters and they were okay, if a bit sluggish. I ended up liking the Shimano rapid fire shifters better than the twist and I also liked the Shimano rapid fire over the SRAM thumb shifters...I just did not like the thumb for up and down shifting. My Cannondale Road Warrior used the flat bar 443 rapid fire shifters and my BIL GT used Shimano Deore rapid fires...no problem with either.
I bought some Paul Thumbies for a while and loved it with Shimano barcons. They were AWESOME. I ended swapping a moustache bar for my flat bar...good bye Paul Thumbies, hello barcons. So far I like the barcons best, but they do position the shifter a bit far for me...no big deal for me.