I can't really answer for how the users react to those. But I can tell you about my experience, not long ago.
I came across a library that does exactly that, and the page had login demo, with just a field: the email. I like VERY much the idea of registering with only the email at first, completing the info afterwards. However, after logout I tried to login again and was presented with the same form.
I got really confused, as if my account had been lost and I had to register again. It took me a while to realize what was going on, and I'm a web developer.
Bottomline: if you're gonna do it, make sure your users know what's going on. And be ware that many users don't have the habbit of opening emails often. This could be an even bigger hassle for them than defining a strong password.