Great things about the game:
-Updates are super frequent
-Devs are super communicative on their steam posts/updates
-Has an atmosphere that is unmatched by anything else
-Mod support
-Gameplay is incredibly intense and allows you to take it as seriously as you want to [as long as you don't start trolling super hard]
-Great Visuals + enemy designs
-Bots were added to allow more people to "solo" levels

Less great things about the game:
-It doesn't have matchmaking [In my opinion this isn't a bad thing because of the amount of information you have to deal with. I'd rather not have to deal with a random person joining and trolling as well. But I'm gonna put this here because I see that there are plenty of people complaining about this]
-Since the connections are p2p sometimes during the middle of a run someone will disconnect or the host will stop responding and everything stops working for a bit
-They made their own discord for people to use in case people want to play without using the bots, everyone who I've talked to from there seemed pretty nice but I understand why most people don't like having to join a discord server to play multiplayer with random people
-After every update they add and remove certain maps [There's a rotation, I'm not going to explain it because honestly how it works seems to be changing all the time and I just didn't keep track of it at all]

The "bad" stuff
-Some levels take a GOOD while to finish and since you can't stop in the middle of it you have to have quite a lot of time to be able to finish some of the later levels
-It is incredibly punishing for a more casual type of player, I'd say that most people who got into GTFO got into it because of the "challenges" that it presented with its gameplay, etc. But for a more casual player this can be viewed as a bad thing, I think. Since there are no difficulty options [even though the first levels are often quite easy when compared to some of the stuff at the end of the game] it can be quite daunting for someone who isn't experienced with this type of game

The actual bad stuff
-Sometimes there are some pretty "intense" bugs, textures sometimes completely disappear, the audio sometimes completely bugs out, etc. Sometimes these bugs can affect gameplay by quite a lot. But usually it only happens for a second or two and then everything goes back to normal [although bugs related to audio usually don't]
-The audio levels are EVERYWHERE with this game, every time I open it I take off my headphones because if I don't I know for certain that I'll go deaf by how loud that opening is, the noises for when you're going down to a level are incredibly loud as well

Conclusion

GTFO is probably one of the best games to play with your friends, wouldn't really purchase it if I didn't know anybody to play with me though.