I have a Zumo 550, a bit long in the tooth now but still works. I know a lot of folk use car satnavs in cases etc, and it can certainly work and be cheaper. Bike specific ones are made for the purpose and just make life a bit easier. Pay your money and take your choice.
A useful Zumo owners forum
here
A couple of friends have Zumo 660 and are hapy with them. One has started using
this route planner which is Google and you just download the route to your unit, whatever the make. He's very happy with it, seems very easy to use (compared to Mapsource anyway). I intend to give it a try but haven't yet.
There are free convereters around which can change POI databases from one format to another, I've done this with no issues at all.