One of those weird instances of style over substance that actually works, is plenty fun, and comes completely recommended. I've never walked out of there less than excited to go back.
I've you've enjoyed tacos outside of Montreal before, don't be alarmed at paying way too much for not-quite-the-best tacos you've ever had. I've you've had fried chicken anywhere else in Montreal (Icechouse, Triple Crown), then really skip overpaying for the fried chicken here. Expect however to just have a great time eating whatever you do end up slightly over paying for.
There's really no other place like Grumman's you can experience in the city. Sure it's a hipster mouse-trap, but it's a friendly hipster mouse-trap with personality and heart. The staff is never less than stellar and pleasant and fun and ready to laugh with you over just about anything, and from the moment you walk in the vibe is that of people having a good time being serviced by people who appear to be enjoying themselves working there.
I've nitpicked the pricing a bit and that's the market in Quebec unfortunately, but the food is inventive and really good (not best) enough that you never get the impression you're actually overpaying for anything. I recommend the extensive wine list, and suggest caution with the cocktails; the recipes are way outside your standard playbook, and I've yet to have one that didn't have me reaching to find why someone thought that would make a good drink. The wine though...