Apparently 980's have been having issues for the last several months with their RF output and a slight latency. I don't know if that's been fixed. I have seen columns of buttons fail on that model too.

MX880 doesn't have all of the fancy boolean logic and as many variables as the MX980 will support but it's a bit smarter than a MX900 and the one I've had for 4 years now seems to be working fine. I haven't seen as many problems with that unit in general.