Had some fun with Hidden Source, but it should be known that HS could be a case study on how not to balance a game. There are so many bullshit tactics it's hard not to just give in and become one of them. It might sound like I'm mad, but I'm not. I'm pretty good at it and on a good day stand a pretty good chance of winning as Hidden, and IRIS is easy.
Let's make a list:
1. Pigstick. The obvious one. Anyone with half a sense of timing can easily wipe the floor by pigsticking. You hit the button, and ~1 second later whatever's in front of your knife explodes. It was made to counter camping, but most people just use it constantly and so it's been banned on most decent servers, which sucks because it's pretty good at its intended use and campers are endemic.
Segue...
2. Camping. Corners, rails, generators, vents, fences, you name it, you can hide there and if you've played a video game before chances are you have the reflexes to kill the hidden before he reaches you. The main counter is pigsticking, but it's been banned in most places, and the places it hasn't are the Wild West, with every form of bullshit tactics imaginable flying everywhere. In any given game, about a third of the players will immediately run to a secluded spot and sit there with a shotgun pointed at the only entrance. It's not too hard to deal with though, most of the campers are horrible and you can pounce right onto their heads and then stab and they have no idea how to respond.
3. Shotguns. Shotguns are awful. Flail around, click the mouse, hidden dies, feel good about yourself. What's sad is that the F2000 is way better if you actually know what you're doing because you can hit the hidden from across the map and it's faster, but shotgun is easy to use.
Still, it's a fun game and a good player (Which I can be when I'm in a groove) can pretty easily counter most of the bullshit, but it's still bullshit and still pisses you off when you get killed by it.