Well, my sister-in-law, tended to buy M-Bs, her old 190TE needed dumping at 6 years, it always had dealer servicing, her husband's C??? sport estate had to get moved on after 5 years, next came a very nice 320E estate, it ended up being a money pit and got binned at end of last year, her husband now has a BMW 3 convertible as he is often away from UK, her next car is a BMW 3??D with £0 road tax. At change over time they looked at M-B - too expensive for what they have become in terms of reliability, Audi - they dropped that idea after very poor experience at Audi dealerships when looking for a car (seems to be the way!), BMW just wanted to try to give them what they wanted when they wanted it.
Another story, a guy at work seems to buy a few cars, he works shifts so money rich, he gets an indie to work on his cars, this indie runs a M-B but handles M-B, BMW and Audi mainly, out of these three marques, he reckons Audi has become the best one for reliability, mainly due to BMW and M-B dropping standards.
I've always, even when being a VW owner, given this advice to potential Audi buyers - only buy an Audi fitted with an Audi engine, this comments seems to run true as most of the reported engine issues are with the 2.0TFSI - VW engine and 2.0TDI engine again a VW engine. My experience of VW - a B5 Passat 2.8 V6 4Motion = Audi based platform with Audi engine and Quattro system, all in all a very reliable car over 13 years of ownership also a 9N Polo 1.4 SE = Skoda platform with VW engine, all in all a money pit in its early years until VAG sorted out some design issues, though a reliable car for 100K and currently 11.5 years daily use.
I've also ran Ford, Fiesta with original 1.6 Zetec engine for 10 years/100K always changing oil (Comma Zetec) at 12 months period, using Comma Easy Flush prior to every other service. That engine never had any issues other than when at about 10 years, I flushed the cooling system and that caused a massive waterpump leak = new pump and another new cambelt!!