I wish they would explain what this GSM shield does apart from the one year subscription and connection to their own platform?
There are several unlocked GSM shields and modules available with working libraries and tutorials for under $50, using the exact same GSM module (SIM900): http://dx.com/s/Sim900?category=499
None of those appear to be Arduino "shields", unless I'm not looking at them correctly? An Arduino shield is plug/play, whereas I would define a generic "module" as not.
There are several unlocked GSM shields and modules available with working libraries and tutorials for under $50, using the exact same GSM module (SIM900): http://dx.com/s/Sim900?category=499