I prefer win7 currently. If you like 7 I'd say stick with it.
If you're on 8 or 8.1, definitely worth the upgrade.
I installed the win10 upgrade within the first 24 hours of it going live. I was plagued with bluescreens after the first week.
The main problem with it i see for most people is lack of control over the automatic updates. Imagine it deciding to download updates when you're in the middle of a wow session. Watch your latency skyrocket if you don't have decent internet.
10 should have better graphical performance. I plan to be on it hopefully for legion levelling. 5x 2560x1600 res windows is graphically very demanding.