Dimmable Ballast

Has anyone used the ones that dim by percent for a single bulb? not the one that recognizes different bulbs. I was wondering how they worked compared to just getting one that accept different bulbs.

i was wondering the same thing, mostly how to tell the diferance when i buy them on line if the 600 watt bulbe can run at 400 or 250 watts or do i need to change bulbs, some place i read that to wait 30 seconds between changing the out poot for the bulb to s cool down