Fixies are good for early season training. Put a low gear on it and it really helps with your spin when you are doing LSD.
As far as using it in traffic, I think its highly over-rated. The only advantage I can see is that you can track stand at stop lights, but risk of pedal strikes outweighs track standing, IMHO.
__________________
Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1