I ran 18" Conti DWS for the past 2 winters with no complaints. Excellent handling and no sliding, and a much more comfortable ride than the OE runflats. A shame they're so good - no more excuses to stay home for snow days. While these are not winter-specific, they get the job done where I live in NJ. I'm sure you get a lot more snow than me though. Winter-specific I've seen many folks go with Nokian with nothing but praise.
This will not mess up your suspension, but should you damage your suspension some other way, the dealer may try to blame it on non-OE rubber. That said, tons of folks on here and many other boards have all left the runflats behind.
640i Gran Coupe | M Sport | Sapphire Black | Amaro Brown Merino (indiv) | Dark Red Sycamore (indiv) | Bang & Olufson
N54 535i xDrive | M Sport | Platinum Gray | Logic 7 | Nav | HD Audio | RDSport Roof Spoiler | Dinan Stage 2 | Dinan Exhaust | KW v3 Coilovers | Beyern Mesh | Conti DWS 255/40-285/35 | RPI Scoop | Gloss Black Kidneys | (sold)