When it comes to T12 and T8 I feel I am knowledgeable, having changed out thousands of lamps and hundreds of ballasts over the years and what not.  But when it comes to T5, this is a whole new world to me.  What advantages/disadvantages do T5s have vs T12 and T8?  Why would someone use T5 over T8? 

Thanks

T5's are more efficient under designed conditions. But with open fixtures close to fast moving air they may loose the efficacy due to being too cold, the are designed for about 35degC air around the tube (because that is typical for most enclosed fluorescent fixtures).
The concept was allowed be the modern phosphors, allowing operation with the high surface power density, so allowing the tubes to be thenner, with less mercury selfabsorbtion.

There is one irony with these lamps:
The official (IEC) lumen rating corresponds to that colder operation, in the enclosed fixtures they tend to be more efficient (IEC requires the output measurement to be done in 25degC air, but these lamps use to be optimized for 35degC), what makes the official performance rating rather pessimistic (about 10..20%, depends on exact conditions).

@Medved: American T8 are significantly different technology than the European T8. American T8 are brighter than T12. European T8 have the same brightness as T12.