The place to go for poutine. They serve about a thousand different varieties, ranging from the classic curds n' gravy to more elaborate creations with things like smoked meat (corned beef) and guacamole.
I had the "La Poivrée", which is like the classic but with pepper sauce. It was darn tasty, but to me poutine is still just something to have in Canada; I'll never crave it elsewhere.
The clientele is a hearty mix of locals and tourists, and there's no shortage of photos or Instagram'ing going on at the tables. All in all it's a good experience, and a great place to go for those visiting Montréal for the first time.