When I first got Call of Duty 4: Modern Warfare I would snipe constantly and it was a lot of fun. I would pick all of the stealthy perks to be as effective as possible and I got more kills than some players did running around with assault rifles.
With the future releases of COD I started to use assault rifles, SMG's, and sometimes LMG's more. With assault rifles I can actually snipe a bit if I'm accurate and my play style is a mix of close encounter stealth and running and gunning. When I use an SMG I tend to never stop moving so that I can get around players quickly and catch them off guard before unloading a clip into them (evil laugh).
Since MW3, especially in BO2, and somewhat in Ghosts I've been using the shotgun a lot. In Black Ops 2 I equip the first shotgun with a long barrel, laser sight, and fast mag. Then I can run around the map without ever aiming, and blast players away and then continue on running and repeat the process. I get A LOT of kills with minimal deaths this way. A lot of times I will look up and shoot a player that hasn't even started firing yet because they see that I have a shotgun, but don't realize that if you aim straight and use those perks, the kill range is greatly increased as far as shotguns go. People tend to get frustrated with this play style and will say that it's too easy, but IT WORKS. It's just an effective strategy and there are times where I get massacred by veteran players so it is definitely not OP. I just tell them that if they don't like it then just get the upperhand and beat me, which they usually don't do and then continue to get frustrated (another evil laugh).