I can almost use this on my phone...but since I can not paste my highly randomized password, from my password database, into their website. Why not? I don't know, I think paste is disabled by Javascript. Who the hell would think that is a good idea?
I can't stand it when sites do that (blocking paste) but I don't think that is what's happening here. It's just their placeholder text "Password" that is messing things up. Try long-pressing just on the left of the 'P' of the placeholder. That worked for me after a few tries (on Android).
Which phone were you using? I don't think this is actually on purpose, but I just tried it on my iPhone and did find that it was inconsistent and difficult to paste a password. I am going to send some emails and see if we can figure out the issue. Let me know what kind of phone you are on.
Nexus 5, fully updated on chrome. The site looks great, and I'm excited to have free mobile spreadsheets.