I've tried it a few months ago on my nexus 7, while it was still in alpha, and it was not usable even on very basic tasks. I can understand that most apps and features were mocks (that was an alpha after all), but the browser kept on clicking 10 or 15 px under where I actually clicked (and I suspect it was a qtwebkit problem from qt5, so that's why I feel concerned : it may not be fixable directly by ubuntu).
Anyway, I'm glad to see this happen - that's the door for opensource innovations in the field of mobile. Having a true linux on my mobiles is something I really want. I also hope they will make the ubuntu sdk portable. As much as I would love to use and contribute to a mobile ubuntu, I don't want it on my desktop and laptop (and this kind of limitation is the very reason why I didn't ever care to look at ios development).