I use a 20W MR16 "spot" (not sure the degrees), and find it quite adequate. Last night I added an amber xenon flasher from All Electronics ($8) and built a little power distribution point with a couple of switches. It's all working well. I really like the strobe. It's bright enough to be quite noticable without being obnoxious.

The only problem is that I'm using a lead-acid cell that's only 2.3 AH. I foolishly thought that dividing AH by amps would get me hours of runtime. Well, lead-acid cells taper their voltage off pretty badly as they discharge, so even with 1.6A draw and 2.3 AH, by 40 minutes the headlight is pretty orange.

Tomorrow I'm shopping for a 4AH battery, and I'll pick up another charger for at work; gonna need it when it's dark BOTH ways this winter.

Here's the battery charger I use. It's designed to be attached permanently to motorcycle or other batteries, and can be left attached indefinitely. It charges fast but then kicks into maintainance mode so it doesn't overcharge. It says $40 but I've gotten them for about $26 from local discount stores. Wally world probably has something like this.

http://www.jcwhitney.com/autoparts/P.../p-894/c-10111