Of particular interest to me is the fact that Mac OS 10.5 has a lot of interesting new technology on the inside. Unlike what I often see in software, this is not just a cosmetic skin over the same old system. Apple appears to have done some solid (and original) computer science research as a part of developing this release.
Assuming reality lives up to first impressions (which I'll know in a few weeks, I'm sure), I'm going to have to get myself a copy. If there are critical bugs (all new products have bugs, but they may not be critical to me), then I'll wait for one or two service packs first.