I've played around with runflats/non runflats since 2005 from E90 to E92 and now E93.
2005, my E90 325i Exclusive pack came with Bridgestone 17" runflats
Negatives: Horrible ride. Every little road imperfection felt like driving over a landmine. Car developed rattles quickly.
Positives: This generation runflats could be driven longer with a flat tire. I drove from Bloem to Kroonstad on a flat tire, bought a R40 plug, continued to JHB.
Non-runflat experience: I replaced them with Michelin pilot primacy non-runflat and ride comfort improved a lot. However, the car didn't feel as sharp around corners.
2007, my E92 335i M-Sport pack came with Pirelli Eufori 19" runflats
Negatives: Pirelli wet handling sucked. 19" runflat availability was a problem. Driven them flat and only lasted 80kms.
Positives: 2nd Generation 19" runflats on M-Sport E92 were more comfortable than 1st Generation 17" runflats on Exclusive pack E90. I also think BMW must have adjusted suspension settings on E92.
Non-runflat experience: I replaced them with Toyo Proxes. Made little difference in ride quality (if any). 2 Toyo Proxes cost R4000, less than one runflat tire. They last longer.
Alternative runflat : Since non-runflat made no comfort gains, I replaced Toyo with Pirelli PZero runflats. Immediately, I could feel same ride comfort as non runflats. Car was just a joy.
2010, my E93 335i M-Sport pack came with Michelin Pilot Sport 18" runflats
Negatives: They are expensive. More than Bridgestones and Pirelli
Positives: Probably the best all round runflat tires. They definitely last longer.
Alternative runflat : I replaced Michelin with Pirelli PZero and still happy. I like the ride so much that I don't have desire for non-runflat.
2014, test drove 435i Coupe and 428 Convertible with 19" runflats
Positives: Ride comfort is brilliant. This car on "comfort" mode is relaxing. However, on Sport+ mode, road undulations are felt but they never transferred into the cabin or steering wheel feel. Looking forward to buying the 435i.