Never used t5 before, have used cfls and other flouros for seedlings/cloning but have always vegged under mh or cmh. (Currently rocking and loving cmh)
But am very impressed with t5s and will be making it my veg light.
Let me put it this way, both tahoes and jedis are forming there 4th leaf sets and are under 2" tall (from soil to top of new growth)
I want to pick up some t5's, I have been runnin my babys under t8's till they're ready for the MH.
T5's>T8's hands down.