I’ve been using the betas on my home desktop since WWDC. Most things work, and I occasionally notice something that’s different. If you replaced Snow Leopard with Leopard, I probably wouldn’t notice for at least a day. (Replace Leopard with Tiger and I’d notice within seconds.)
When I use my laptop or my work desktop, both still running Leopard, I don’t miss any Snow Leopard features.
I preordered it from Amazon, but I probably won’t get it until next week, even though everyone else can get it on Friday. This doesn’t bother me at all, and I’m not going to put any effort into getting it sooner.
Some minor things are broken with certain applications, and I’ve hit a few minor bugs with the OS. Therefore, I don’t plan to upgrade my work computer or laptop for a little while, possibly until 10.6.1 or until application support is a bit better.
We’re likely to see software that takes advantage of new Snow Leopard APIs in the next few years. But there’s no reason to rush the upgrade and be among the first to hit compatibility problems. Day-to-day use is almost unchanged from Leopard. That’s a good thing, but it won’t make for a flashy release.
Upgrade when convenient. No rush.