Connections is a big one for me, especially if you arrive into certain airports like MIA where certain flights are full of people that take > 5 mins per person to process. Not anyone's fault so no offence intended, but if I can avoid standing in a queue where it generally took < 2 mins to be processed after waiting a very long time for others to be processed at much slower speeds.... And esp with connections in the US onto AA - if you are in J/F, you miss your flight due to imm...and you need to get the next flight, good luck with all the upgraders filling the F/J cabins.
Paulwuk makes a good point re traveling with others. Would you leave them...? Otherwise manord, definitely get it. You will appreciate it when you are faced with a rammed immigration hall with no one at the GE machines.
And i do wonder just how many people will sign up. I bet some airports will have GE lines whereas some other never will (im betting MIA wont - prog has been open to Americans for a few years now and still no queues....very much a tourist airport that one...)