Leopard on my powerbook, Tiger on the iMac C2D at work. I really don't like Leopard all that much, though. It's a lot slower and glitchy compared to Tiger but I'm too far along to go back.
I find Leopard chews up RAM and processor power and I don't really see any benefits. Safari 3 is also a complete dog at the moment. In comparison to Tiger there's also not a single feature of Leopard that I use that I wasn't using before so, given the decrease in speed and glitches, overall it's been a negative experience so far.
During the week I find using the Tiger-iMac is refreshing. Sad, but true.
Hopefully they'll tweak and refine it more with 5.2, but I'm not expecting much.