I ride the Catrike 700; reputably the fastest non-custom trike out there. The 700 is very aerodynamic, very light and it has that huge 700c wheel in the back for high speed power. Plus it just looks cool:
The one difficulty (and this is true of amost all trikes) is that there are a couple parts that you can't find in a regular bike shop, specifically the 16" wheels up front. Now this is no huge deal, it is just not as easy as going down to the bike shop and picking up a fresh set of tires and tubes. It takes a little foresight, shopping (which is actually fun when you're shopping for bike parts) and planning.