Actually the release numbers are year/month So 9.10 is 2009 October. The next release will be 10.04 then 10.10, 11.04, 11.10, etc. Every 6 months another free upgrade. This can even be done in place, not changing anything until you reboot. I can keep working/playing while I upgrade the OS I'm using. Then a reboot, all my settings are the same, and I'm using the new OS.
Each new upgrade is approx the equivalent of upgrading from W2k to XP.
Have a look at compiz animations on youtube.
One thing which has always annoyed me about MS, and they have only just rectified in W7, and as far as I know has always been available in *nix, is the ability to change the order on the taskbar.
EDIT:
BTW its also free
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