I had the chance to play with a Z10 recently and I must say I was quite surprised by how polished the experience seemed.

a. The OS felt as responsive as Windows Phone and all the custom gesture recognition works as advertised(e.g. Peek).

b. The apps I used included Twitter, the Blackberry Hub (e-mail, contact etc.), the Camera app and Evernote. Of these, the Evernote app seemed to be a little rough (mainly UI glitches), but the rest worked well.

I haven't used the Developer Toolchain for QNX/BB10, but have friends who seem to think that it's not too bad. They are waiting for sales figures to decide whether or not they should think about releasing a BB10 app.

Off-topic: As someone who likes tinkering with things, hardware repair-ability is something I pay attention to. The Z10 is the most repairable phone out there according to iFixit (8/10): http://www.ifixit.com/Teardown/BlackBerry+Z10+Teardown/13445...