As has been pointed out you get:
a more comfortable flight
near certainty of being able to stow your carry-on in the rack
two checked bags at no extra charge
some catering (and the meals seem to be improving)
more priority in IRROPS (at least that is my experience)

AA value the bump up at $0.10 per mile for voucher compensation if you buy an "instant upgrade" ticket and IRROPS force you into coach. I feel that it is generally worth that much and will sometimes pay more. If you are willing to pay what it costs to give it a try then go for it!