Build 10240 is on the right track, and thankfully as MS have de-coupled the first party apps from the OS in terms of updates so bugs in Mail and other apps can be updated via the store in the background pretty quickly.
Can't say I've used Win10 on a small tablet, but I have been testing it since Jan on a Linx 10, and in the last month I've also installed it onto my Dell Inspiron hybrid laptop/tablet device.
On both of these devices I've actually found it to be incredibly stable. I haven't been able to replicate the issues mentioned in the article, however I have come across a couple of problems elsewhere. (e.g. I can't edit the TCP/IP properties in the GUI for a VPN connection...)
Overall it seems like a big improvement on Windows 8.1, both for touch and non-touch devices alike. I haven't recommended people to upgrade from Windows 7 to 8/8.1 as I didn't feel it brought enough to the party vs the changes in UI / learning curve for most people. However it looks like W10 will be easy enough for end users to get used to (compared to 7) and brings enough goodies to warrant the upgrade.
UI aside (everyone is fixated on the UI, which whilst important is only one aspect of an OS) Win 10 feels like a big jump, particularly from Win 7 in areas of performance and manageability. The revised areas of start screen / store / apps from Windows 8/8.1 is a huge relief, and now makes enough sense for it not to be a 'blocker' in terms of recommending the platform to end users.
Whilst the first party apps aren't the best, not in any area to be honest, they are mostly good enough for the majority of users. Yes, Mail is crap compared to Outlook or even Outlook.com or GMail, but it's good enough for basic email use. I use Outlook 2016 preview on my laptop, signed into the domain, but have my MS account hooked up and use Mail for day-to-day operations on my personal mailbox.
There's some massive gaps that MS need to address. OneDrive placeholders is a huge issue for me, Groove Music needs some basic features adding (MP3 tag editing, seamless playback etc.) and Edge needs extension support (coming soon but not soon enough).
However ignoring Windows 10 on it's own, and looking at it by comparing to Windows 7 and Windows 8.x then it looks very promising. The servicing model of the applications means they should be getting regular updates which is overdue and will make a huge difference based on the updates done during the Insider Preview programme.
Think I'll give it 6 months, see if there's anything major reported and looks at rolling out a small pilot. Genuinely does look like the best release (at RTM) of Windows and the future roadmap looks equally promising.