The ratings will tank (to be clear, I am a fan of Fallon) and Leno will come back in 3 months.
But serious, I think Fallon will do great and be received well. I think Conan's schtick was a little "too crazy" for The Tonight Show.
I find either people love Conan or hate him. There's no middle ground. I actually like Conan's interviews over his bits.