I installed Mac OS X 10.5 this evening. I knew it could be a mistake (I've been using Macs since the first one was released in 1984, and have been punished for being an early adopter many times), but the list of disappointments I was expecting proved to be too short in one critical respect: having installed 10.5, my Mac started freezing every time the Finder loaded. No way out, except to power-cycle the machine. After much experimenting and checking of vendor web pages for firmware updates, I discovered that the Sonnet Tempo-X eSATA 4+4 card, which I depend on to access the vast majority of the storage on my Mac, is not compatible with 10.5. In the final sentence of the paragraph describing the latest firmware update for that card, Sonnet does acknowledge this: "Note that Mac OS X Version 10.5 beta will not currently operate with these cards and Power Mac computers." Wish I'd thought to go searching through vendor web pages for these tiny, vital gems of information before I'd installed 10.5. But even more, I wish Sonnet had fixed the problem during 10.5's lengthy beta period. (My past experiences with the company lead me to expect better than this from them.) So, if you happen to have one of these Sonnet cards, best to hold-off on the 10.5 upgrade, and maybe even drop Sonnet a line expressing your interest in a fix. I know I'm waiting anxiously for their response to my email.
Late addition: It also seems worth noting that Apple's practice of releasing products like 10.5 on Fridays (and in the late afternoon, no less) leaves customers who encounter problems like mine stranded without access to tech support from most vendors for more than two days. Monday releases would be a vastly better choice in this regard.
As an article in Mac User magazine, I think it was, said of early adopters about twenty years ago: "you can tell the pioneers by the arrows in their backs." This is a hopelessly flawed summary of events in the settlement of the American west, but a very good description of the settlement of many technological frontiers. Of course, if my judgement had been better, I'd've skipped being an early adopter in this case, as I usually do, but it remains to be seen, from Sonnet's response to this problem, whether that would have made any difference.
Much later addition: This issue has been resolved.