To the other answerers: $20? Where are you buying it for $20? My stupid browser shows $30.
If you aren't convinced, forget it for now. It doesn't make your life better unless you need those navigational features. About half the reason to buy it is for the gestures. If you don't have a glass trackpad, it won't activate the advanced gestures anyhow.
A few compelling features that really go beyond eye-candy and navigation ease:
-- improved address space layout randomization (ASLR) -- a super-amazing security improvement. If you already agree that OS X in general is far above Windows, this will knock your socks off.
-- recovery partition -- so much better than Time Machine for fixing big OS issues.
-- vertical text -- just for us who type in Chinese and want to represent the old style of text.
-- versions -- like on-the-fly Time Machine.
Other ideas were already available to some extent in third-party form: Auto-save.
Just for the Windows-lovers: Full-screen Safari. Oh, I'm trembling with excitement.
Next month, if you change your mind, you can still buy it-- but it likely will be OS 10.7.2, even better.