I've had a set of zipp 808 tubulars for the past couple years that I've been racing on. Over the winter I got myself a set of Enve 6.7 clinchers. I ride these wheels full time (training and racing). During the beginning of the season I still just raced with my Zipps, but I've switched to racing my Enve's full time and keeping my Zipps only on the TT bike. The quality of clinchers has really come a long way and I don't really feel any difference between the two. I've heard great things things about Williams and would have gotten them myself had I not got such a good deal on my Enve wheels. I have a teammate that runs the 3.4s and loves them and another friend that just got the 4.5s and has no complaints.