I rarely use it, I find a lot of perks more helpful then attachments, like what's a the point of long barrel when you can simply get closer with Lightweight or Extreme Conditioning, extended clip with Scavenger, or Quickdraw with anything but LMGs (I find the ADS quick enough)
Flak Jacket: Survive multiple rockets and grenades exploding by your feet, scratch from a knife to your shin insta-kills you, seems pretty legit.
I run it almost exclusively.. Depending on the guns.... Quickdraw, stock and sight. Quickdraw, stock and fastmags. Quickdraw, silencer and fastmags. Quickdraw, fastmags and a sight.
You see the one thing in common for these... I use to not run quickdraw at all and run with dexterity thinking that was enough.. ever since I switched to quickdraw AND dexterity it has made a BIG difference.. I don't really use that combo on SMGs though.. quickdraw isn't needed.
I knew FMGs would be OP before the game came out: http://www.gamefaqs.com/boards/995751-call-of-duty-modern-warfare-3/59204323