I picked up a new Madone 4.5 yesterday. I have a perfectly good Trek SLR1600 but I do believe it is too big for me (56cm) so I tried on the Madone at 54cm and it fits much nicer. The carbon fiber is much nicer on the road, less jarring and in general just feels smoother. I got the compact crank set 50/34 and the 11-25 cassette. So far I think I'm going to like that set-up as well. The bike handles great, lightweight and responsive. The only complaint I have is the Bontrager Race saddle, which is the same one that I had on my 1600. It soon will join the one from the 1600 on the shelf as I replace it with the Specialized saddle I bought a couple of months ago.
Having a bike fit the way it should makes all the difference in the world. Today I only got in 18 miles but I did 63+ yesterday on the 1600 so this was just a day after ride for me. Over the next few days I'll try to get out and really run it but my first impressions are very satisfactory.