Last Christmas Mrs. Bruin wanted to buy me something cycling related and since I have just about everything else, I asked for a TT helmet. I got a Rudy Project and I like it but I don't think it's a HUGE advantage.

I've seen wind tunnel data that looks good (i.e. MIT) and others that say it's not so good ...especially when you have wind that is not coming directly into your face (I think this came from Discovery Channel's tests). In the real world, I believe it offers a small advantage.

The heat factor is one thing to consider...I wore mine in the Honu Half IM in Kona this year and at one point I was really hot so I dumped water over my head only to remember that I didn't have any vents (what a jackass ) I ended up squirting water into the front vent and it helped a little.