Its worth trying the DD-WRT alternative ROM, my 615 was rubbish with the firmware it came with, couldn't keep VPN connections up at all, needed to restart it every couple of days becasue it wouldn't accept connections, but since the the DD-WRT firmware on its not missed a beat, its been brilliant, so the hardware is decent just rubbish firmware. its worth trying, it won't cost you anything, and if it doesn't work, well then go buy a Netgear or whatever, put the stock firmware back on and sell it on ebay!
Mike