Would that be from a BMW dealer or another manufacturer? Personally, I wouldn't buy one from a non-BMW dealer. Extended warranties usually are a crap-shoot either way.
If you can get it Certified at a BMW dealer, that may be your best bet if you are the original owner or bought it at a BMW dealership to begin with. Will usually run $1500-$1700 to do so and will extend the original warranty to 6 yr/100K mile coverage. Only downside is you have to pay $50 per warranty fix, but that is relatively cheap compared to paying for the complete repair.