I made a purchase from Zenner last year and was very pleased with their service - they were quick, responsive to emails (in English) and the parcel arrived via DHL in a couple of days, as most items from German dealers do. Based on my experience I would recommend them.
Modell-Land are good, of course, and I've made several purchases from them - but their surcharge (5%) for using Paypal annoys me a bit, they are the only German dealer I've yet come across who does this. However their prices are generally very good as Alan says, so maybe this is one way they manage to keep their list prices so low...... They can (in my experience) also be difficult to get email responses from, though once you actually place an order their service is excellent.
Other German dealers that I have personal experiences of and can recommend are Steba.org (Stefan Bauer), Baumann Modellspielwaren, Hunti's Modellshop (Annelie Huntemann) and Der Lokschuppen Dortmund - all of which happily take Paypal with no surcharges.....
Jon.