Not a Z4 or Z5, but I really like my Felt ZC frameset (built w/ Sram drivetrain and wheels and "fit" parts I had on hand). I've been using it for two full seasons now and it is working really well. I had a more aggressive frame before that (Cannondale CAAD7) that I got to fit me OK, but with the Z I was able to get a better fit, and got there without using extreme rise stem. I really can't tell you if the CF, as a material, is actually more comfortable than the Cannondale aluminum (I had upgraded with really nice fork, and well-fitting bars, stem, saddle, seatpost, all of which I transferred over to the Z) because the fit is just a little better. The Z is definitately more comfortable and even with the slightly more upright position, I feel (and objectively seem to ride) a little faster and just feel better in the long run.