Oh I agree! It's a bit like the Hunter Killer Drone when gamers call it a free kill, yet if you ask how often people use it there's a chance it won't have a kill to use ratio above 1. Same applies to the 'place and forget' lark with Betties and Claymores but there are ways to avoid and counter them but it still won't stop people moaning.
Yeah I rarely use the HK unless I just want to run low streaks. I've looked at my own stats with it and I think it only gets kills around 50% of the time.
I love c4's. I always get a kill them, unless someone has flak jacket on, which makes you invulnerable to 2 of these lol