I need to use my machines. So I'm the kind of geek who likes to, barring the addition of a new non-critical machine, wait 6-18 months before switching major releases of OS X.
As it happens I am going to buy an iMac in the next few months, but for now I've no hands on experience with 10.6. Still, if you review the late Aug 2009 late Aug 2009 OS X related reads the dog whistles are loud and clear.
Snow Leopard breaks stuff. Lots of stuff. It's also slower or only minimally faster than 10.5 on most machines, and Apple blew their major security feature (memory randomization) -- they obviously couldn't get it to work. So Windows 7 has better fundamental security -- as does Vista for that matter. Resolution independence? Oh, you remember that from 10.4 days? Of course not.
The only good news is that you can (illegally) install the $30 of 10.6 over 10.4. Considering Apple's long tradition of abusing early adopters I give everyone my ethical permission to do so. It's only fair.
There's good stuff in 10.6, and there's bad stuff. (For example, it looks like Apple continues to wreak havoc on pioneering concepts in the old Mac Classic file system.) There's enough good stuff that I'm looking forward to running 10.6.1 on a non-essential new iMac. Otherwise I strongly advise waiting 6 months before updating -- and even then you should confirm that your current printers and scanners and so on will work with 10.6.
Unless, that is, you're feeling lucky.
Update: This is the best review so far.
Update 9/6/09: I played with Snow Leopard in the Apple store today. As others have noted, it's hard to find any differences from 10.5. From what I read at least as many things have been broken as have been fixed. Unless you have to upgrade from 10.4, or you're buying a new machine, you shouldn't consider Snow Leopard before March of 2010.
Update 3/13/2010: I was too optimistic. My 10.6.2 machine crashes hard frequently. Among other issues, Apple screwed permissions and firewire. As usual.