I've heard from a few people that an HID conversion in a reflector type headlight designed for halogen bulbs sucks. What I'm having a hard time wrapping my brain around is that if the focal point of the HID bulb is in the same location as the focal point of the halogen bulb, how could it suck? How does it suck?

I have a '98 Tacoma with a fairly sharp cut off and relatively wide beam spread with the stock headlights and Silverstars and found some "bi-zenon" (I guess that means high and low beam from the same bulb) HIDs for about $300, but am a little wary of them, with what I've heard.