the lumen per watt output is higher. someone posted a comparison a couple weeks back. think of it the same way as a vho vs. a mh lamp. a 110w vho bulb puts out approximately 3000 lumens, while a 175 watt mh lamp puts out approx 15000 lumens. not sure exactly on the stats for the t5, but i do know its is a higher number of lumens per watt than vho (but less than mh), giving you more light in a smaller bulb diameter, and more luments per watt consumed (for those who are power bill conscious). also side by side you can fit almost 2 t5 in the same space as a t12, so for in hood installations where you need actinic, but cant fit a vho, this is the way to get your color.
i saw an order form for one laying on the desk at my lfs, so they are about to break out here in the states, up until now there were but a few who imported them from europe.
someone from europe can prolly give us more info, i know barricuda uses or has used them