I ate here every day on my most recent trip to Chiang Mai. It's not fancy, but it's good, flavorful vegetarian food for super cheap. You can get two items from the lunch buffet with rice for only 35 baht, and they have incredible khao soi for 35 baht as well. I've had veggie khao soi from a couple of other places, but I think this place has the best one, and it's by far the cheapest.
Yes, the portions are a bit small, but it's so cheap that you can just order two portions if you'd like. And the older lady who works here speaks great English, so no worries about any language barrier.
Also, it's pretty close to Muang Mai market, which is a great place to stock up on delicious tropical fruits! Definitely the cheapest market around.