When it comes to his in-ring work, I think John Cena is average, however with the right opponent he can be absolutely phenomenal. Of course, the right opponent is pretty much always an above average competitor, but it's never the case of Cena being carried, it's just him rising to somewhere near their level when they face. Somewhat unfortunately, these opponents are few and far between, leaving most Cena matches as simply average bouts at best. I do say "somewhat" because it makes those matches even more special.
Mic skills-wise it totally depends on what Cena we're seeing. If it's serious Cena then I'd honestly place him as one of the best. Would he make my top five, doubtful but still, he's seriously good. If it's joker Cena, the Cena we got most of the time it seems, we are in for same crap. I'll admit that he can occasionally get a chuckle out of me, but a lot of the times it's just crap. Of course, I'm only talking about what's currently relevant. His rapping gimmick and even pre-PG to a lesser extent aren't really being taking into consideration here since they are no longer an accurate representation of Cena's mic skills.
Finally, the backstage stuff. One, considering Reks is the main reason he's no longer employed in WWE, I highly doubt he has any reason to lie about Cena. So, I'm not doubting the validity of his stories, but I'm doubting why they should have any impact on my opinions of the guy. Firstly, it's wrestling, that's just how it goes. Unfortunate yes, but rarely anyone can be that successful without stepping on a few toes. However, there is enough evidence to counteract those negative comments directed towards Cena or better put "he does just enough/more good to balance/outweigh the bad".