Is it just a good router that you need or is the phone side of it necessary too?
It's both, but the phone side is an advantage. One of the things that appeals is that AVM appear to support the FritzBox for longer than most router manufacturers.
I could go for one of the Draytek models with a phone option, but I think Fritz do it better.
The one thing that does rule them out for many people is that given the fall in the pound relative to the euro, they are getting to be particularly expensive, and appeal to a limited market. Possibly one of the reasons nobody has taken up a UK dealership. Zen probably get a decent deal through bulk buying and see the Fritz as part of their sales proposition.