Re: the Claymore... I mostly only use it on the Human Soldier and dodgeless classes like Batarians and Krogan. Not just because they make reload-cancelling super-easy, but also because the Claymore just "feels" right on them. Batarian Soldier with Claymore might actually be my overall most effective class on this game. I sometimes use it on the Human Infiltrator too.
By the way, I tried going through a single-player campaign, planning to see what it's like to take Adrenaline Rush with Bonus Power and use Marksman as my extra power, with the Claymore. That was the ENTIRE POINT of doing the SP game. The thought was that I would shoot the Claymore, use AR to get a second shot, and then use MM to get a third shot and sustained accuracy and damage bonus with both AR and MM active. That plan was quickly short circuited when I discovered that MM doesn't give you a free reload in SP, and for some reason you can't use the Bonus Power of AR to activate Marksman. What, Cloak+Marksman is OK, but AR+MM would be OP? Super disappointing. Also, I hadn't played SP in a long time, and SP Insanity is ridiculously easy.