So, I've upgraded 3/6 of the eligible PCs I've got in the house. All three started from Win7, two from Home and one from Pro.
PC #1 - AMD APU Home Theatre PC. No problems at all here. Upgrade took a long time, probably due to the slow processor, but has made the machine faster, and it hasn't had any stability issues at all.
PC #2 - Intel Haswell General Desktop/Server. One minor problem - had to wipe and reinstall VPN software, even though it reported as upgrading fine. Otherwise, no problems at all.
PC #3 - Intel/NVidia laptop. Fairly large problem - Optimus (Switchable graphics from integrated to discrete) no longer works. Computer does not recognize NVidia card anymore, although I can turn it on and off using a hardware button. Otherwise, no issues, and machine is faster.
For me, the issue with PC #3 isn't one, because it's my previous laptop and has a dead battery and sits in my living room permanently running off power and isn't used for gaming. For other people, there's a chance it could be a real problem, although given it's a slightly quirky 1st gen Optimus laptop from the Win7 days probably is the cause of the issue. I also haven't done much troubleshooting on it, because, well, it doesn't change what I use the laptop for at all.
If you have a PC that's a single chip (AMD APU/Intel with no discrete graphics), you're almost certainly safe to upgrade. If you have a more complex rig, I'd recommend waiting until the big feature/patch release hits in September at the earliest. Likewise, if it's your only machine and its working fine, wait. Windows 10 is a nice upgrade over 7 and 8.1, but it won't change your life (unless you like talking to Cortana a lot).