I've been running the newest build on my Icon all day and finally it's stable enough that I think I can use it on my daily driver. I can log into online banking, copy and paste is more reliable, the store doesn't crash quite as often, and the overall performance is a step up from the last build. Thankfully I had a PIN set on my lockscreen when updating because I was stuck in the eternal "loading..." hell with the start screen, forcing me to enter the wrong PIN enough times to cause a factory reset. I didn't load a backup and it's been running surprising well from a fresh reboot.
Still a lot of rough parts to the build though. Cortana is still inferior to the 8.1 version, I can't ask it to find me a location and take me there, the maps app just seems to crash. Some third party apps seem to need an update since they don't run properly. Also I know this has been discussed before, but I'm not a fan of the new mail app, it's definitely a step back from the app used in 8.1. All accounts are forced into one app, can't have multiple different mailboxes, and I have yet to figure out if you can even view all mailboxes together like you could in 8.1. The formatting options are nice but having more freedom to how you manage your multiple accounts is much preferred IMO. Copying and pasting in some apps is still broken, like from OneNote.
They are on the right track, but you can tell this OS is still months away from completion. I know they are planning a major update or updates to Windows 10 desktop post release but I hope with the core version releasing later this month, I hope this means they can then focus more on working on the mobile version.