Garry's Mod takes Source and makes a dynamic, moddable, enjoyable game out of it. However, it's slightly underpopulated.
Garry's Mod's true glory lies in it's modability. It's easier to mod than any other Source game, and thus is modded more than any other Source game. Some servers run mods that completely flip the game over on it's side.
Unfortunately, finding a fun, populated server can be difficult. First of all, almost nobody ever plays Sandbox. Secondly, the most populated servers are RP servers, which, like any roleplaying mod, are populated by whining three year olds who can't seem to get it's a game - and the roleplay mods are quite dull as it is. The Zombie Swarm (or survival? I dunno) mod is decently fun, but finding a good server is rough. There's also a sort of murder-mystery/detective game type (Trouble in Terrorist Town) which can be fun for a while, but is ultimately difficult to learn and, like roleplaying, often has servers with uptight users or (worse) admins.
The game itself is well done and potentially fun, but this is more of a "What the heck, let's make a catapult out of columns and containers" game than one that you will continue to come back to. The ultimate failing of Garry's Mod is that it's almost entirely community based, and the community doesn't do a great job of making it fun.
Although, it's a great tool for posing models if you want images for a Source map, or a comic, etc.