I've been there a few times, and I think I'm safe recommending it.
I'm no expert on German food, but for what it's worth I thought it was tasty, and there was enough of it on the plate. Prices are reasonable, the atmosphere is relaxed and not too stuffy like a lot of places around the Westend, and the staff are friendly and helpful.
The place does seem to fill up after 6 or 6:30pm during the week, though. Last time there (and I think the time before as well), all tables inside were reserved, so if you're thinking of checking it out, give a call ahead of time - the morning of has been early enough in my experience.
Bockenheimer Landstraße 87, 60325 Frankfurt
Tel: 069 74309070 · Fax: 069 74309071
http://www.kunze-frankfurt.de
Mo-Fr 11.30-1.00 Uhr
Sa, 18.00-1.00
U6/U7 between Bockenheimer Warte and Westend