Garry's Mod is great! A lot of opportunities can be had thanks to the props, useful tools, fun maps and a huge library of mods or "add-ons" as they are called.

The props are amazing. This isn't a sandbox like Minecraft but more of "construction" sandbox. You can make cars, shelters, weapons and many other things. Also, the game doesn't take itself seriously so you can have a few laughs thanks to tools like the Faceposer (effect pictured above).

Another great thing is the multiplayer. While there are traditional gamemodes, like Sandbox and Prop Hunt, there are others like Cinema, where you can join other players to watch a movie/video together.

One down side is that the developers aren't that present in the game's development, in my opinion. While Facepunch Studios do put out certain updates regarding security and other minor bugs every once in a while, nothing is what you would call "major". Another downside is that the game is archaic graphically. The props are mostly from HL2 and other old Source engine games and the maps haven't had a major update for a long time.

A suggestion would be to 'modernise' the game. Maybe add some newer props, or maybe a new gameplay feature, or maybe something else. The game, for the most part, has been left alone due to it's age and also due to the studio's newer projects. This shouldn't be the case as there is still fun to be had in Garry's Mod and a few updates can turn this forgotten gem into something new.

Overall, this game is excellent but does suffer a bit due to its age. It's a recommend buy from me!
While the game is cheap without discounts, it can be had for up to 75% off (I purchased it for just 2.49 EUR.) during sales!

EDIT (2nd March 2018): Garry's Mod doesn't go for below 4.99EUR anymore so it's more expenisive. Still, it's rather cheap for what it offers

EDIT (25th November 2018): Garry's Mod does goes below 5 EUR but not frequently. Look out for the best deals and sales!