It's up to each person to decide whether to upgrade or not.
As far as Windows 10 formatting the drive, only if you do a clean install. An upgrade leaves all programs and files intact. Still, best to backup important files first. In my case, I imaged my drive first. On my test tablet, I imaged the drive, replaced the drive with a new one, and restored the image to that, then tested.
Also, don't forget you have 30 days to revert back to Windows 7 if you don't like it.