I've been using the Windows 10 previews since the first surfaced, and use them almost daily, but not constantly throughout the day. My job also involves regular use of other Windows versions, both desktop and server, from 2000 up to 8.1/2012 R2.
I haven't used the latest build (10074) yet, but I will do so tomorrow.
To be honest, I think the Windows 10 UI is shaping up to be another misstep. Now, I hated Windows 8 on release, but since 8.1 and particularly with the monthly updates towards the end of last year, it's improved immensely and I find that the Start Screen is a very effective application launcher. It's actually difficult to go back to 7 once you get used to it, and I now exclusively use a fully updated 8.1 on my own desktops. Never looked back.
Windows 10's UI on the other hand seems to be trying to combine both approaches and not succeeding too brilliantly at either. It's Start Menu equivalent is missing the functionality that made the old Start Menu really great, and by ramming in the Tiles the amount of on-screen real-estate available for collecting and organising the programs you really need to use most often is compromised. It's Start Screen equivalent loses it's effectiveness by again compromising on the available real-estate and by being too "busy".
This is all subject to change as more preview builds emerge, of course, but with the projected release date, what we're seeing now must surely be pretty damn close to what Microsoft are actually going to ship. On the other hand, news like the re-emergence of Aero Glass in build 10074 does indicate that late changes are still possible, and it's always possible that once I get down to using it every day my opinion may change.
Overall I think they're shipping too early. I think that taking another few months to polish the experience, instead of rushing to put out a first-cut that's inevitably going to be rough around the edges, would have benefitted things. I expect that the first few months of updates will deliver what should have been shipped, but I wonder if the damage will have already been done by then. People have very high expectations of Windows 10, especially after the bad rep that the initial releases of 8 got, and in it's current state I just can't see it delivering.
It's also the case that Microsoft are very heavily pushing integration between desktop and mobile in this version, but yet the current previews of Windows Phone are even rawer than the desktop previews - there's just no way that's going to ship until much later this year. So the promised integration is just not going to be there from the outset.
So I wonder why they're shipping so early. The original indicators were that they would ship later this year - say Q4-ish - but current indicators are that it's going to be July. The only explanation I can think of is that something pushed them, and I wonder if it was the upcoming Vulkan vs D3D12 battle. Shipping in Q4 would mean that we'd probably have a Vulkan spec and the first drivers already out (I expect them at Siggraph) some months beforehand, which would give Vulkan a headstart. Whatever it was, it's going to be a shaky first few months for Windows 10 unless things really come together very quickly over the next month or so.