When he's actually taking feuds seriously, he's fine. But it's when he does his whole 'all smiles, narrate the feud for the kids, joking around' bullshit, that's when it becomes painful. His goofy, for the kids, character is ridiculous and I can't see how anyone can enjoy that.
I mean, we all enjoyed that Star Wars promo, right?
I blame the writers though. One week they have him actually take a feud seriously, and the next he's smiling away like he's on all the coke in the world and it doesn't do him any favours.
I actually enjoyed most of Cena's segments in the run up to Wrestlemania this year (even though it was all pointless, as he said shit like "I have to win this match", loses and comes out like "AND WHAT" and it's all swept underneath the booking carpet). He was in his srs character with his srs face on and it was a nice change.