Garry's mod is one of the best sandbox/community driven games on PC and possible in Gaming.
If you are going to buy this, you should buy CS:S with it since many game modes use those textures in it.
Sandbox mode is nice, you can do many things in that but many people also love to play on game modes, I would recommend TTT as a nice game mode if you find a good server but the sky is the limit in Gmod gamemodes(actaully it is the engine but eh).
There are so many things to be said about this game and this review would go on for pages if you were to explain every single custom gamemode that is on garry's mod and you will get you 10/5/2.5 euro or whatever the price is out of it since there is constantly new content developed on servers.
9/10, a great game with a good community if you know where to look.