I have found that riding the track bike (48x16) on rides and commuting has built my leg stregth and ability to push large gears, (my spin is good too, but I've the rollers to thank for that ). But my long distance fitness and core stregth have suffered. The fact that school is kicking my arse might also have something to do with lower the ideal fitness level.

Riding fixed definitely keeps your legs in decent shape though...