I'm with Rich on this one! I saw a really good deal on a MET TT helmet, and was sorely tempted for about 5 seconds, until I realised that I didn't have the engine to justify the bodywork. I would personally be embarrased to race in one and get overtaken.
Conversely, they do save you time, although I believe you have to be going at a fair lick before its noticeable. So if you've got spare cash and want to maximise your performance, then why not.
It all boils down to which of the following 2 maxims you prefer:
"All the gear, no idea" or
"He who dies with the most toys wins" :-)