I have just upgraded my Lumia 830 through the Upgrade Advisor app.
The upgrade build is 10586.107, which is strange because Microsoft specifically mentioned that the .122 and .164 builds made improvements to the upgrade experience for Windows Phone 8.1 users. However, once .107 is installed there is a further update to .164 to be downloaded and installed from Settings. The entire upgrade from Windows Phone 8.1 to Windows 10 took just under 3 hours in total, which was much longer than I was expecting.
One of my biggest concerns was performance. I am relieved to say that this is largely a non-issue. The aging Snapdragon 400 and 1GB RAM is not performing as well as it did on Windows Phone 8.1, but nor it Windows 10 a terrible experience either. Apps that loaded instantly in the past (eg. Messaging) now take a second longer, and this is a theme throughout the entire operating system. In addition to this the lockscreen still exhibits that painful delay displaying the time more often than not, which is an issue which shouldn't affect a modern operating system in 2016. I have also noticed that some apps (notably Groove and 6Tag) do not display an icon on the All Apps list and are instead an accent coloured tile. This was also something I really hoped would have been solved.
One of my favourite new features is the ability to reply to a message straight from a notification without having to open the app. The new cursor option on the keyboard is an equally great addition as well, but there are several areas of regression. Cortana isn't anywhere near the level of polish as on Windows Phone 8.1 and the entire user interface looks rather hideous with white cards against a black background.
Camera performance seems largely unaffected, but it does take an extra second or two to open. The Camera app will open instantly, but it will remain black until it has finished loading. The Photos app on the other hand suffers from some real performance issues with thumbnails taking time to load and zooming be a less than smooth affair. Hopefully some app updates will fix these issues.
I am not going to debate the pros and cons of upgrading to Windows 10, because they are too numerous to even begin to discuss. If you are happy with Windows 10 and its direction, then it makes for a decent upgrade on the Lumia 830. It performs fine and the phone remains usable, and some of the new features are really nice. If you want to find out more about my experience you can read my blog.
I will not be updating my own Lumia 930 to Windows 10 because I don't feel it is ready, especially with regards to the camera and video experience suffering. However, this Lumia 830 is used intensively as a daily driver by another family member and so far it has made for a solid experience with no serious complaints.