Before I get into this, let me explain that I would classify myself as a "fast recreational rider". My work schedule keeps me from training as I would need to be competitive. Despite that, I own 3 bikes. My "rainy day" bike is a 1998 Trek 1000, redone a couple years ago with Ultegra and running Spinergy Spox. I have a 1999 Trek 5500, completely redone (paint and all) with D/A, FSA carbon cranks, and I just replaced my Rolf VP's with FSA RD600's. I also have a Trek Team Time Trial frame with a HED disc rear, tri-spoke front, HED carbon TT bars and everything else D/A. Same course, TT first, then 5500, another day 5500 first, then TT, comparible winds, etc, I'm about 2.5 mph faster on the TT bike than the 5500. I haven't timed anything with the new FSA RD600's, I had the Spinergy's on the 5500 when I did the timing a few months ago. If it's "aero", you'll be faster. You may still fall off on the hills (like I do, it's pretty flat here in Dallas), cause "aero" doesn't help you there, but you'll have an easier time keeping up with your buds, and be able to expend less energy for a given speed in any case. OK, not very "scientific", but I know I'm working less and going faster on the TT bike. - Chip