I do own a ez5 myself, so just to add my observation
Hardware-wise, it has no serious problems I can see. But software they left much to be desired for, it was only until very recently did they resolve a very serious save corruption problem with their firmware. In all honesty I can't say the ez5 has had a great launch. Until the recent expansion pak, the gba-support ez is well-known for has very little to do with ez5 itself. And note that the expansion pak uses NORflash instead of any PSram at all, so the gba support isn't very attractive if you want to switch between more than a couple of gba games at any one time; a slot-2 gba card would be a better solution for that. Just be prepared to get what you pay for, mainly the rumble and expansion memory, and ds-gba link