I used to fit a lot of Viessmanns, I moved away from them simply because of the price. I haven't had a single one break down, and still service most of them regularly, they're running perfectly and looking in great condition.
Positives are: stainless steel heat exchanger, easy to work on, decent components used throughout.
Negatives are: Expensive (inc. parts), a lot of plastic (although never had a problem), rubber hoses (again never had a problem but the potential is there).
If you're happy with the installer and he likes fitting them then go with it. A boilers reliability is down to a good installation, not brand reputation. I normally fit Ideal Logic+/independant + now for what it's worth.