Did have a play about with a Z10 in my Voda shop for half hour (hardly long enough to give a thorough opinion).

First off, they could have rounded the edges of the casing, felt a little uncomfortable to hold. Build quality felt good, pretty much felt the same weight as the N900 (only about 50g difference, I think). Liked the idea of the interchangeable battery and the fact they've managed to keep the size of that to a minimum, so no Galaxy slab syndrome.

As for the UI, I found it quick and fluid, but half an hour of up-time wasn't exactly taxing things. Some of the UI effects are nice, especially the fade to menu from the home screen. Although one of the first things I did notice and found disappointing was there was no back button, so I ended up pressing the lock button twice and starting from the homescreen again.

Would I "up"grade to a Z10?
If it wasn't for the fact that I'd be losing quite a few nifty little tricks of the N900, I would do. But only after the device has been on the market for at least 6 months, give it time to settle and watch to see if it does something to impress within that time.
Otherwise I'll be staying with the N900 and waiting for Ubuntu, Firefox and Jolla.

Don't think I'll bother with the Q10. That is one ugly beast and too "old school BlackBerry". Now if it was a Z10 with a slide out QWERTY..............