The deal-breaker for me, at this point, is lack of Classic (System 9) support. I still have a few old apps that I don't want to get rid of totally but aren't worth the cost to replace or upgrade. If I ever get desperate for a new Leopard feature, with Time Machine the most likely candidate at this point, I may create multiple partitions, or I might just put off the process until I go Intel and have to lose the System 9 apps for that reason anyhow.

BTW, here are (within what is otherwise a very positive review) Computerworld's "Five reasons not to make the jump to Leopard - yet."