I did upgrade my Lumia 630 DS to Windows 10 - The latest build. Worked fine most of the times.
The issue is that Windows 10, while being quite stable right now, just can't keep up with the 512 MB RAM. The basic features work alright. You can text and call and nothing goes wrong there and Groove works okay as well. But other apps crash after being used for a few minutes.
Take 6tag (instagram client by Rudy Hyun) for example. It works okay for a few minutes but while uploading photos, it sometimes crashes. Same goes for Edge. Browsing and viewing light web pages goes okay but downloading and especially uploading is a tiresome procedure, often taken several tries before getting right. And just forget about the new Facebook and Messenger applications. They don't work at all. If you're lucky enough that they even launch, they'll crash in a matter of seconds.
If you're just going to be using the phone for basic usage, it'll work fine. Better than 8.1, in my opinion but anything apart from that, you'll get frustrated soon.