I reckon the diablo would be good for me as a do-it-all light. I agree with jimjam - if I sometimes use the full power then it's not wasted, even if it's on medium most of the time. I don't mind switching it, although I'd probably want to get a remote switch so I can turn it down when a car comes the other way without taking my hand off the bars (with the halogens I have a thumb switch to turn one of them off). Fast road riding on the moor could do with more light than my current setup - I have to slow down a bit in the winter because I can't _quite_ see well enough with 25W halogen to feel totally confident.
There's an Exposure dealer fairly local to me so perhaps I'll drop in at the weekend and see if they've got any on demo. I'm still not sure if a Joystick would already be brighter than what I'm already using, and whether the Diablo would just be overkill.
What I do like about the separate-battery lights (like that Lupine wwaswas mentioned) is the lower profile (lamp on the front, small battery on the back) rather than sticking up on the top like a wireless one would. But not so convenient to swap between commuting bike and helmet as an all-in-one unit...
Oh, @reggiegasket - well, I've got four, but you're right, one at a time