Weight - and sidewall stiffness are the issues I've heard - personally I've never had runflats on a car - I have driven cars with runflats and they feel like concrete overshoes. ; -)
Oh and after my OT comments on the 'vette - three things I don't like about the 135i:
- Runflats (easily rectified by several burnouts and then new tires ; -)
- no LSD and a question about being able to drain and fill the gearlube in the diff
- Weight - was hoping for no more than 3000 lbs, but that's a little complaint