Fixing windshield cracks has been going on in my area for maybe 5 years or so. I have never needed it, but others have told me it works. MyHarley is right, get it done ASAP so it doesn't lengthen with heat, stress, etc.
If you do replace the windshield, make sure you get a BMW OEM and not a cheap replacement. I have no proof, but from experience with a Volvo, I think that the OEM windshield is better quality than a cheap $150 replacement.