After the upgrade, everything seems to be working except two obscure utilities: ClickBook and TiVoDecode Manager.
ClickBook is essential to the production of Bento, and didn't even launch under Leopard 1.5.6. I bought an upgrade to a new 4.0 version, which works, but has some problems with the 4-up layout we use. (It lays out each page in the order 3, 4, 1, 2 instead of 1, 2, 3, 4 as you'd expect.) I've sent in a support ticket.
I've been using TiVoDecode Manager 2.1 to archive The Amazing Race to DVD. Unfortunately neither 2.1 nor the Leopard-specific 3.0 version works on my computer, and from what I've gleaned online I'm not the only one. I even tried using 2.1 on another Mac, still running Tiger, and it failed there too (!?). Searching around online I found a number of alternatives, including iTiVo, which is based on the TiVoDecode Manager code base but is under extremely active development. So far it seems to be the best replacement, though I'm still trying to find a combination of parameter settings that does exactly what I want.
The TiVo thing took up the entire afternoon and well into the evening and, well, early morning as well, because each download-and-decode attempt takes one or two hours. I did manage to get in some writing time, though; one must have one's priorities.
Apart from those two problems I'm generally quite impressed with Leopard. Time Machine has already saved my bacon once; I accidentally deleted a whole directory of files and was able to immediately recover all but the newest one (and that one can easily be reproduced). Leopard FTW!