I've only ever watched Jimmy Fallon because of The Roots. In my opinion, if he didn't have them, his ratings would be nowhere. Nevertheless, I think he'll do okay with The Tonight Show. In many ways, he reminds me of Johnny Carson in his younger days. Johnny Carson grew into the icon he became, but when he was younger, he was mad-corny too.
All that being said, I haven't watched The Tonight Show on a regular basis since Jay Leno retired the FIRST time (...back when Kevin Eubanks was still on the show) Late night TV used to be the only game in town, but it's no longer the case now that we have the Internet and smartphones..