As per Transam, really, I used to fit them exclusively.
But, although the product is well enough designed/engineered/straightforward to fit/commission, the local rep(s)seem to me have been conspicuous by their absence (OK, so I only fit say, a dozen year - so not a major installer then.....).
Also, the techhelp desk service is intermittent, and given that 'I' sell the product to my customers, I am not keen on the way that Vaillant/Homeserve then latch onto 'my' customers and bombard them with insurance/boiler service contracts which effectively cut me out of the deal.
so, rant over, the product is fine, follow the MI's to the letter, thoroughly clean and treat the system water as if you want to drink it and the customer gets an appliance they deserve - after all, it ain't the cheapest on the market.....
DH