Stash is a pretty cool and unique place. The atmosphere is cozy and the piano player at the entrance is a really nice touch. My coworkers and I stopped in on our last night in Montreal and it was a solid last meal.
We ordered a couple of appetizers since we had heard that the pierogis and the potato pancakes were good, but didn't want to get them as our entrees. We tried all three types of pierogis and they were all excellent. For an entree I ordered the Golabki (stuffed cabbage rolls) and they were very tasty.
Overall, I'm giving it three stars because of their tough competition in Old Montreal. It was a good experience overall but I don't think they quite stand up to many of the other restaurants in the area. I likely would return if I were specifically craving Polish food, but otherwise I would probably head to one of the other restaurants in the area.