Scott made mention of T-5's having a 'cleaner' light. I know, that's not an easily defined term ... but it's still true, regardless.
Comparing 400 watt MH to 5-tube T-5's, in actual use the overall level of lighting seemed identical. That is, the side of the shop with the T-5's did not seem any brighter than the side next to it, with the MH. Yet ... on the T-5 side ... the light seemed 'softer,' colors clearer, shadows lesser. You just seemed to be able to see things better - especially when doing detail work on a bench.
The power savings are very real, as well. The same light, or better, for about 1/3 of the electric bill. Other advantages were a lesser load on the air conditioning (HID gets HOT!), and the fact that the fixture never goes completely dark. That's what happens when the only bulb or ballast in a fixture goes bad; the T-5's I saw had 5 bulbs and 2 ballasts.
