I've had my new 5k iMac since the end of November and its been great, no complaints, up till now that is. Just last week, the Magic Mouse 2 that came with the iMac started having issues tracking, that is I moved the mouse to the left or right the cursor would jump a little and then not move at all. I'd move it up and down, and it may work a little then stop. I changed mouse pads, and that really didn't much, but here's the odd part. I removed the mouse pad and using it on a bare desk. It now seems to work. Perhaps I'd chalk it up to the fact that it didn't like the particular mouse pads I was using, except for the fact that I was the same mouse pad for over 2 months problem free.