If you buy the GBA Laminated 3 inch IPS screen kit with the laminated-IPS-ready GBA shell, then this USB battery should definitely be on your shopping list. I assembled the 'NEW' Funnyplaying GBA screen kit and proceeded to use IKEA rechargeable AAs as before, but noticed that the battery warning light would come on almost straight away, especially with screen on bright. Perhaps the low-voltage threshold is reached quicker with the laminated mod. The USB battery seems to have solved this. Oh, and it fits like a glove in the IPS-ready shell. No cutting or trimming required.
For comparison, I have another GBA with an earlier non-laminated IPS screen running the IKEA rechargeables which doesn't suffer the same low voltage issue, and lasts for ages on the AAs.
Get a clear or frosted battery cover so you can see the charge status lights without taking off the battery cover. The PCB looks great - put it on show!
I'm not sure why the heatsink must be applied over the battery charge lights guide on the PCB. Was it an afterthought?