After a couple of days with OS X 10.6 aka Snow Leopard I have to say that it is not worth bothering with. It breaks things. Nothing major, but there are no rewards for that hassle. ZFS would have been nice, but it is not there. Applications that would use more than 2/3/4 Gigabytes of memory would have been nice. But even the recently updated FinalCut is using 64bit yet to address this much memory.
All those glorious changes under the hood? Well, all I notice were the things that are broken. I am so glad that I kept my main machines at 10.5