Just FYI I had a hard drive formatted to ExFat as well for my Windows 7 side and Mountain Lion side, and I constantly ran into problems. Had to recover data 3 times.
I suggest creating two partitions on the drive, one NTFS (for Windows) and the other HTFS+ (for ML) and then use Paragon NTFS on ML, and Macdrive on Windows to have full read/write access to both partitions from either OS. I've been running it like that for months without any problems. And you can still easily back up the HD to another drive using the same partition scheme.
Just my two cents