BMWs have a reputation for being tremendous performance vehicles. However they also have a reputation for being incredibly unreliable. 
Is this reputation deserved? No not really. These vehicles are incredibly reliable for the first several years and 50-70k of mileage.
Things substantially change after this point. Due to the heavy usage of plastic components and increasingly complex sensors these vehicles now have substantially more problems in their later lives than previous models did.
This is why I highly recommend that anyone interested in a new BMW should strongly consider leasing it instead of buying the vehicle. You will save lots of money because BMWs depreciate so quickly after leaving the lot.