Most tube screamers have a sharp upper mid hump, but the FDII is smoother and more natural sounding. It's also extremely versatile, because of the mini 3 way switch. You can use that switch to dial it in with certain guitars or amps.
It's a fat sounding pedal, so it doesn't thin your sound out at all. The boost can be used to add a bit more volume to the 1st channel, or it can be turned up to simulate a fuzz pedal. The 2nd channel has a lot of gain, but it's not shredder type gain, it's more Hendrixy and fuzzy.
I'd definitely recommend grabbing a 10th Ann. Mosfet version right now, while you still can. You can't lose. Pay $200, use it for as long as you like, and sell it for $250 when they're gone.
Hey, did you ever get your Klon? IMO, that's the be all end all. That's the only pedal I'd never ever sell.