When in Montreal - je mange poutine!
It's a tourist attraction, so visit for that if nothing else. I'll review it on more than just it's reputation.
First, it's a ways from anywhere. The closest metro stop is a hike away.
There will probably be a line. We only waited about 15 minutes lunchtime on a Sunday. When we left the line went around the building.
It's not a great deal. The large classic poutine is close to $10. That's a lot for fries, cheese and gravy. I didn't think it was that large either, but maybe it's because I'm used to American sized portions. Prices go up from there. It was good though and there are many varieties. (But I've had better)
It's cash only, which we didn't find out until we had consumed a lot of poutine and a bunch of beers