I preordered Tiger and didn't have any problems until i switched to Intel (noticeably less stable and more buggy) I will be pre pordering Leopard and installing straight away. Really looking forward to the first release that is going to be Intel from Day one.
I think the Tiger experience was very variable depending on what you used with it (software-wise). I had a not-as-pleasant-experience with pre-order... it was buggy for about two or three months. By about 10.4.3 it was fine, IIRC. Definitely a lot less than a year before things went back to normal.
If you get lucky in terms of what software you're using, Leopard will likely behave quite well from 10.5.0. If you get unlucky, it may be difficult. If you want to maximize your chances, you can always be over-conservative and reformat your drive, install it "clean," and be very minimalist about only installing software on it that you need and that you know works on it. Then you're also very likely to be fine from day one.