OS X 10.7 – Lion – is available in the Mac App Store today. However, based on my (limited) testing, I'm not in a rush to upgrade yet. I installed the Golden Master from the developer downloads earlier, and I was not thrilled.
Oh, I love the new Mail. And I got used to the reversed scrolling pretty quickly (you can set the scrolling back to the way it was on 10.6 and below in the System Preferences if you like). I really like the fact that you can now resize a window from any edge (welcome to 1995, OS X!). The dialog boxes seem to pop out at you, too, and I like that. There's an emergency partition for reinstalling so you don't have to try and find your install disk (but still, have a bootable backup of your existing system!).
What I didn't like was the fact that every time I tried to do something, I got a beach ball spinning. I finally had to hard boot out of it. Now, this is in limited testing – I need to go back and test for real – doing a test migration of my data and programs and seeing how that works, testing with better hardware (I did use a low-end disk, as it was all I had at the time), and checking that the critical programs I use every day work OK. Some, like SuperDuper, have already issued updates for Lion.
My point here is not that there is anything wrong with Lion. My point is that you shouldn't rush into upgrading. Make sure anything you have that you absolutely need to run works OK. Use a backup disk for your first install, and then either do a migration after a clean install, or backup your system to the disk and do the upgrade there. Even after you're satisfied, keep a Snow Leopard backup around for a couple of weeks or so just in case.
Personally, I don't think I will upgrade before the inevitable 10.7.1 update is available. There's always a danger in going to a x.0 release of any operating system – no company can possibly test for every combination of hardware and software available. There will be a lot of things that just don't work right, but probably for a very small number of users. But if you're one of them, it's suddenly a big problem. So, as we've said here many time on this blog, backup, backup, backup! and then test an upgrade before committing. Yes, it's more work, but it's a lot better than digging yourself into a hole you can't get out of.