This year I added a '93 Trek 5900, my first carbon fiber bike. It's not completely original, with some Shimano 600 components rather than the original Dura Ace. Fine with me. The fellow I got it from only wanted the DA goodies. I only wanted the frame. It's still a great bike. Even with the 600 rather than Dura Ace, it's still at least 5 lbs lighter than my steel '89 Ironman, about 20 lbs vs 25 lbs.
Sooo... any major improvements in speed? Nope. But the fellow I bought it from told me not to expect too much. It rides almost exactly like my Ironman, but lighter. It's from an era when carbon bikes were built pretty much like steel bikes -- geometry, etc. -- so they didn't look or feel radically different -- just fatter tubes, no externally visible lugs or joints. In contrast the newer Tarmac I test rode last year felt dramatically different, like the difference between a BMW sedan and a Miata.
On a good day and a route with lots of short, steep punchy climbs (we don't have any long climbs here), the Trekenstein is good for 1 mph faster than my usual average on the Ironman on the same routes, same conditions, etc.
Or, I can ride the same speed as I would on the Ironman, with less effort, and ride farther or be less tired after my usual 20-30 mile workout rides.
So the lighter weight is definitely an advantage. Not a huge advantage -- a better engine would make more difference. But it helps enough to make it easier to keep up with some spirited club rides. Especially on climbs.
But the heavier Ironman is more comfortable. It's nearly a perfect fit for me, and I've tweaked, polished and massaged it to wring out as much mechanical advantage as I can within reasonable cost effectiveness. So while it's not quite as fast as the carbon bike over 20-30 miles, it's more comfortable over 50 miles so my times work out about the same over distance.
I'm a little too stretched out on the Trek 5900, due to that nifty but ridiculously long Ibis titanium stem. My neck was injured being hit by cars twice, so comfort is an issue on longer rides. If I can get the fit where I need it, and the Trek is as comfy as my Ironman, it's possible I might see significant differences in speed or finishing times over longer distances.
I like having both. I don't really care whether one is faster. It's just nice to have a choice depending on how I feel on any given day.