I use a 400 watt MH. In my case it is a necessity, as the 30 story highrise across the street blocks out all but indirect sunlight.
It's been almost 3 years now, and I have had great success using HIDs. I have bloomed phals, masdevallia, oncidium alliance, brassavola and my one lonely paph.
At first, heat buildup was a problem. Now, I have 4 fans synced up to my timer. Also, in the winter I keep the heat in my grow room set to low and in the summer I set the AC slightly cooler than the rest of the house. I'm thinking about buying a light mover, which would double my usable grow area. BTW HIDs are not halogens. It is not even the same technology at all.