There really are some fantastic user levels, and of course you can create your own, which I find great fun as well.
Some user levels will force you to use tricks you may not have thought of (Sorting comes to mind). Perhaps most aren't as great as the official levels, but there are 548 published user levels, so you're bound to find dozens of levels that you truly enjoy. Hundreds more are accessible through URL but not published in-game (I just built levelId 1802) so URL hopping should keep you busy from now until about forever, and thats if you don't participate in the forum games or build your own levels for others to try out.
I'd say its a tremendous value if you really enjoy the free game.