It depends a lot on where you live/drive, whether there are good independent tire shops convenient to you and how risk adverse you are.
In Boston, it's not a bad deal, particular when we have a bad winter like the last one. We didn't have it on our now departed 330xi and spent $750 in tire replacements in three years (cost for insurance at lease inception was $500). We have it on our 335 vert and are already ahead $75 after one incident ($675 to replace a runflat tire and rim after an encounter with a pothole). I'm still deciding about the M3 as I was quoted $995.
The other thing to check is what the maximum the policies will pay out. A lot of the tire store policies have limits of $350 per incident and a low overall cap. The one that we have for the 335 is no limit up to $3000 for the policy. The M3 limit will be higher (finance person checking on this for me).
BTW, I don't have insurance for the X5 with 19" wheels - my experience is that the low profile tires are more of a problem.