Just a note about how happy I am with the latest OS X release (10.9 a.k.a., Mavericks.) My computer is fairly old now (2008 MacBook Pro), and it's been really struggling under my usage scenario (Safari open with at least 20 tabs - and sometimes 50 or more! - along with TextMate open with at least dozens of files, plus at least Mail.app, iPhoto, and probably iTunes hanging around in the background.) Safari was the main culprit, eating up huge amounts of RAM (I only have 4 GB which is the max for this old machine,) and beachballing frequently. Very annoying. Mavericks has almost completley elliminated this problem. It's like getting a brand new machine. And the upgrade is free from any Mac running at least 10.6.8. Very cool.
But a word to the wise: backup before you update. Mine went smoothly, but I had a total meltdown updating b.'s machine. Ended up having to completely wipe the disc and reinstall (thank god I had the 10.6 DVD!) If I hadn't had the Time Machine backup I might well be looking for somewhere to sleep tonight. Also: RAM is cheap! If you have a Mac with less than 4GB of RAM you really should upgrade it. 8GB (2 x 4GB sticks) costs $88. Totally worth it. If you only have 2 GB it's really not enough.