Using 10581 in my Lumia 830. It's mostly good, they have made a massive improvement in the past few months since I last used it, even fixing the text rendering bug introduced in WP8.1 Update 2 (at last!). My favourite part is edge, which was a complete mess in the last build I used. The speed increase from IE in 8.1 is very noticeable.
1. Screen blinks randomly (turns black for half a second, then back to normal). I can't figure out what causes this, but it blinked about 20 times during a music sync in Groove Music.
2. Some app tiles lose their icons. This happens on both the start screen and apps list. Re-pinning to the start screen fixes it, but I haven't found a solution for the missing icons in the apps list.
3. The Store does not check for updates automatically, and the 'Check For Updates' button finds no updates (it shows the icon that it is "working" for about 2 minutes before finding zero updates). The only way I can update any apps is to manually search for them in the app store, or use an edge link for system apps. Has anybody got a solution for this?
4. Windows Camera is rubbish. Photo quality is poor and the 'Rich Capture' option is straight up broken. Sometimes it gets stuck on 'adding the finishing touches' but never actually does anything, which results in a 720p photo with a 'change lighting' option which fails to work. When "adding the finishing touches" it does seem to work, the image created is far too dark, and the option to change lighting doesn't appear... It needs serious work. For now, I'm using Camera360 Sight, which is doing an OK job at creating a usable image.
5. Alarms & Clock app contains no sounds other than "default" which is silent.
6. The PIN entry system on the lock screen is ugly, especially if you are using a lock screen with text on it (which appears underneath the PIN text, making it look a mess). Instead of being transparent; it would be better if the screen moved up (like 8.1), or the PIN pad was coloured (or grey), or if the background becomes blurred.
7. Using the Dialer keypad is excruciatingly slow. Most likely because its searching for contacts with similar numbers, which is nice, but I would prefer a dialer keypad which functions quickly.