The SIG 556 is on the nasty ugly Prohibited list in Canada. Pity because it is an outstanding piece of engineering. Without a doubt better than anything made or designed in North America. Every little design detail shows intelligent thought. The legal civilian version is probably just as good. The only trouble is there are none of the aftermarket parts available, like there are for the AR.
Maple Leaf,
Either you forgot to add the smilie to your comment above, or you must've misread the Model number, 'cause there is no way in hell anybody would describe that American made abortion, the SIG 556, "
as an outstanding piece of engineering. Without a doubt better than anything made or designed in North America. Every little design detail shows intelligent thought." 
I'm hoping you were referring to the SIG/Swiss Arms 550/551/552/553 family or their related cousins, the Classic Green/Black Special/Blue Star..etc... The SIG 556 is a pale imitation of the former and built to a far cheaper price point in the US.
As for the original poster, in my experience, STAG ARs are much better built than the Bushmaster rifles and I have yet to come across a bad one. Walter, from ArmsEast is the primary STAG importer in Canada and fully stands behind his product.
The direct gas impingement system of the AR can be dirty, but for civilian use is works spectacularly well. Most civilians simply do not shoot enough in the run of a year to make this an issue. If you were deployed to Iraq and shooting cases of ammo a week on full auto in sandstorms, then keeping your rifle immaculate would be an issue...not so much for Canadian shooters at the range.
As for the latest "fad" to hit the AR world, the piston upper, most systems work fine with a little tweaking, but I really don't see the point for us hobby shooters. Some of the systems out there aren't made that well and are unreliable during the best of times, others require constant tweaking and the systems that are reliable are almost prohibitively expensive. At the end of the day, civilian shooters aren't going to realize any additional increases in performance/scores by using a piston upper. Sure, it is cleaner than the direct gas system, but when we look at the total volume of rounds expended it really doen't matter what system you're using to the average shooter....but, if you're just looking to buy into the high "Look Cool Value" then, by all means buy one!
If you're a lefty, the left hand STAG is a great buy. The couple of left hand shooters I know who have purchased them really appreciate having a dedicated left hand rifle for them. Sure, they can and still use right hand rifles, but the lefty is more convenient and intuitive for them.
