I guess this is the "big daddy" of Stockholm vegetarian restaurants. It's certainly a very nice place, and a large property (by Stockholm restaurant standards), though correspondingly crowded.
I went for a lunch buffet and was not overwhelmed. There were decent choices, some tasty and some healthy, but overall I was not awed by the options or by its diversity. There was a fair bit of generic or of the sloppy, indifferent over-sauced variety. You could carve out a tasty and decent meal, but it was by no means one of the best vegetarian buffets I've been to.
The location is spectacular, with great views over the water and town, just up the cliffs of Södermalm from the photography museum. The view alone is worth it on a pleasant day. Perhaps that means they don't work quite as hard on the food as they could. Also, if you're traveling alone, be prepared to figure out what to do with your belongings as you make multiple trips indoors to the buffet.
Cons: not inexpensive, food is decent but not spectacular, seating/safety of bags a bit of an issue