I finally have a bit of what I consider to be a comparison between using and not using my disc/deep wheels on the TT bike.
So this weekend was the TT championship, its really the only long one around (unless you do triathalon) at 40K - most of the stand alone TT's around here are around 10 to 12K.
My hubby and I went out and pre rode the course the day before, since it was supposed to rain hard (ended up dry!) and we didn't want any suprises. The pre-ride I used my TT bike, but had my training wheels on it (Velocity Deep V's). Now we didn't do the whole ride at TT pace, but we did a few fast sections.
What I found was that I really noticed the disc when there was a cross wind. I've always heard that its actually an asset in a crosswind - unless of course its a really strong crosswind and blows you over...., since it acts like a sail - much like when a sailboat uses a crosswind to tack and go faster - and I really do think I could feel the extra boost. Doesn't do much for you in a direct headwind, but the course was really twisty so there were few sections going directly into the wind.
So I definitely think the disc works well, though it's not the lightest wheel available, for relatively flat courses its more of an asset than a hinderence. I wouldn't use it for anything hilly - mainly because of the weight, but also because its older and an 8 speed, fine on my TT bike on the flats and small rollers, but I would want something bigger than a 21 for real climbing