General consensus (from Logic Pro users) is that Snow Leopard is a good upgrade. I have heard of relatively few issues for Logic users to do with Snow leopard.
Do a quick search here for 'snow leopard'. I believe mtec is testing Logic 8 and 9 with SL and has found it good so far.
I should mention most users who have upgraded to SL have reported their machines are running slightly more quickly!
But, as always, I would recommend cloning your Leopard system first before upgrading to a new OS.