I just wanted to clarify that I live in Florida, so there are no hills :) My training partner was having trouble keeping up with me on the flats, uphill (up-bridge) and downhill (downbridge), basically the whole ride. The interesting thing was that my HR was about 3 beats lower than I normally do that ride in and I was going faster. I'm pretty new to all this, and like I said, this is my first set of race wheels so I haven't done any testing but like you said I'm faster on the Nimbles than on the Velocitys. They also make my bike look a lot cooler, which we all know is the most important thing :) Fortunately I have not had any problems yet, so I haven't had to contact Nimble directly (got the wheels from a friend) but I have also heard that they are very helpful and professional.
I have to say that I have learned quite a bit from this forum. You guys really know your stuff.
Mark
Fit2Tri Multisport
http://www.fit2tri.com