I've used Leopard quite a bit, and I think "it's just another OS", but with some useful features. It's cheap compared to Vista, but still expensive. I use Ubuntu, XP, Tiger, Leopard, and Vista. I have to say I like them in this order.
1. Ubuntu
2. Leopard
3. Tiger
4. XP
5. Vista
Ubuntu is a great OS, you can dual boot it easily, and there are tons of programs for it.