Pesonally my favorite game I own.
You can probably tell this because it says: 21.7 hrs last two weeks / 413.3 hrs on record
Garry's Mod is a first person sandbox game, that has almost no limitations due the to workshop community.
Because of the amount of game-modes, it is almost impossible to get bored of.
DarkRP, Trouble in Terrorist Town, Prop Hunt, G-Mod Racer, Gun Gaym (Yes it is spelled like that), Apoc, Melon Runner,
Just to name a few.
But do keep in mind, if you do not have really fast internet speed, or major patients, you may not like this game.
Generally when joining servers, it takes a while to download all the workshop item, skins, and textures.
Also, to play most game-modes such as DarkRP, or Apoc, the game requires other games such as CS:S or HL2 to play, but the textures needed can be downloaded off the internet.
Personally, I give this game a 10/10. Do I think it is worth the price? Hell yes.
I would be willing to pay around $40 for this game.
I would reccomend buying this with keys from a game dealer on steam, or buying it during the Steam Summer Sale, just because it has dropped all the way down to $2.50 before.
As I said erlier, this is my favorite game, and definantly worth the price. Get buyin, steam community!