This game used to be one of the best multiplayer games for its wide ability to modify your gameplay the way you like and enjoy creative addons and plugins made by the community for a different experience in each server.
It had all sorts of fun, from playing with the engine itself and explore its capabilities with cheat-free servers to a huge organized competitive community. The things in between are countless, Roleplay, Fishing, Co-op and much much much more!
But as time passes games tend to die, and that is what is happening right now in this following game. It is pretty much unsupported anymore by community developers and ValvE in most cases.
It has a low number of players, quite limited I would say in each server. It is not your typical first person shooter, it is quite difficult to master and get used to in all ways there is.
Blending in with a server's community might get you to enjoy it a bit for the first few weeks/months , but then you will realize that you have been playing with the same players over and over again with a few number of newcomers playing with you.
So if you would like a game to enjoy playing with people, feels alive, supported by community developers and the game developers themselves, play competitively and even create content for it (maps,skins...etc) this is NOT the game you would want to buy.