I think with T5s the type of fixture and cooling provided with it is crucial.
I had a run of the mill T5 fixture and it didn't have any cooling fans in it and the bulbs started changing really fast. It just had some ventilation holes in the fixture and that was supposed to provide the cooling!
I used the same bulbs in the ATI fixture, which has fans in it for cooling and they lasted much longer.
That's not to say that they last forever. Changes are still needed. But my point is don't skimp on the fixture because you'll be paying it in bulbs...