Love them Love them Love them Check my journal, I use a 250watt T5 with gull wing on one tray, and a 150 watt on another for seedling and younger plants under 30 days. I started a scrog recently and used a 150 watt T5 in my flowering chamber to help them stretch alongside the two 150 watt HPS lights. My ballast has hit a max temp of 86.7*f and thats being taken right on the ballast for the T5. I do have fans though, lol, a lot of fans. I say they are wonderful, if you can get T5HO which are even better.
Are you looking for a specific answer? Do they compare to MH and HPS as far as lumens go? Answer, they can. If you look in my journal you'll see me using multiple T5's for veg and multiple HPS for flower
I use a 4 foot 6 lamp T5, and it's awesome. At the center (sweet spot) a T5 is just as good as a Metal Halide, but overall an HID will give you better light and coverage. However, the T5 uses lees electricity and stays cooler.
Each 4'blub is 120 watts is it not? Which would make a 6 bulb setup use 720 watts which is more electricity then a nice 400w MH/HPS setup. I would only go with T5's if I had heat issues IMO.