We've had our Worcester Bosch combi 8.5 years now, we get it serviced every year to keep it under waranty, the only thing to have gone wrong with it was the timer a few months ago, but that wasn't too expensive to have done.
When we had it fitted. the company recommend worcester bosch, as with a service you get 7 year guarantie, and they're reliable. The guy said they could supply cheaper ones, some less than half the price BUT, they only had 2 year guaranty and he said sometimes some of the cheaper ones they have trouble getting the parts. So I guess it's like a lot of things, do you want to risk having to buy a brand new one every couple of years, or spend a bit more and hope you get your money's worth
We had to have central heating installed, so they measured up the rooms, sized the radiators, and worked out what size boiler we'd need bearing that in mind, and number of people in the house.