I went here last year with Bridget M. who talked a lot about Godspeed You! Black Emperor and I tried to look like I knew what she was talking about.
For a place that focuses entirely on vegetarian food, there's still plenty to be enjoyed by omnivores. Ardent carnivores (we all know them), will likely be disappointed but for them there is a great beer list that rivals anything in the area. The small menu has a number of unusual and wonderfully fresh sandwiches. The food seems remarkably cheap - especially for the area.
I had a lot of fun with the converted cigarette machine that dispenses mini-CDs of weird music you've never heard of. The music is seriously weird stuff, but some come in hand-made containers that are art in their own right. Maybe I'm just a sucker for machines that take my money and give me things.
The interior is cozy, but a little dark for lunch. I suppose given this place is more of a night-life kind of place given its enormous calendar of concerts the dark interior shouldn't be an issue for most. Casa del Popolo is definitely a great and unique place to go if you're passing through Montreal.