I went through this myself, and FWIW, chose custom steel.
Here's why and a wee bit about me. I like long, fast rides. As in 60-120 miles, averaging anywhere from 15 to 18 mph. I like hills, charity rides, and an occasional Gran Fondo. I also like a bike to have a distinct feel and personality, and have become a bit of an environmentalist.
I rode a lot of different bikes. At 41, I grew up on steel road bikes in the 80s, and have also owned aluminum, and most recently a steel production bike (Kona Kapu). I rode carbon from both trek and specialized (the latter a tarmac over 3 days and almost 200 miles). At 6'3 210, I'm not small and I am not a weight weenie.
Bottom line for me -- the carbon bikes didn't feel right. There was a cool precision to them which as a gadget fan I appreciated, but frankly didn't like. The Madone felt fine, but just that -- fine. The Tarmac felt cool, precise, and almost aloof. I got the feeling that it was very cool until the next model came out, and then it would just be last year's bike. How do owners of iPhone 3s feel right now?
I rode less steel, because I had owned it. The Kapu is a great ride, but almost a touring bike and a little flex and a little slow to turn. Some Italian steel bikes felt much more lively, and almost whippy. And I didn't notice the extra 2-3 lbs over the carbon. I also knew that custom bikes could be very highly tuned, and ended up ordering one from Ira Ryan. It is a machine with a real personality, and it is exactly what I had hoped.