It destroys the GPS, you can't use Apple "official" updates through iTunes, you void your warranty, etc.
This. There are many downsides to running 06.15.00 and I am not willing to accept these in return for the ability to use my iPhone for 6 weeks. If I can't get the unlock "bug" to work with my Orange Israel SIM card, I will use my friend's Tmobile US SIM card when I see him at school tomorrow.