I'd agree with Ninjilla, $7 is worth it. CS:S is pretty simplistic compared to TF2 (IMO anyway), but it's good for when you just want to play something quick and to-the-point with simple objectives. Full support for bots on all default maps also makes it easier to play offline than TF2.
If you have and enjoy Garry's Mod, it's also worth it just for all the content you can get for that.

Many,
many custom GMod maps (including a lot of really, really good ones) use CS:S resources...
It also has de_haunts, the GREATEST FPS MAP EVAR (even if only for the theming), available for it. :O (It actually plays okay too, from what I can tell in my limited CS:S experience anyway.)