Originally Posted by alraicercsu
the weight of the rim is another important factor, if you are doing crits aero isn't a big deal as much as having low rotating mass. Road races and TT's are were the more aero wheel pays off.
A slightly different take, at least as far as TT's go:
Believe it or not, the best TT upgrade is the helmet. This came from the "Liberty Seguros: Winds Of Change" article from PezCycling News. "The drag difference between a vented road helmet and an aero-helmet is 2-4 times larger than the difference between a good aero-wheelset and a 32-spoked wheelset."