I am no fan of John Cena, but I have gained a very grudging respect for him. But they gave him the belt at the absolute perfect time. They developed Cena the way that a champion should be developed.
Cena came in hot, had his heel run, turned face and won the US Title at Wrestlemania 20. Cena had, in my opinion, a great run with the US title. He had some good matches against Jesus and Carlito and I was actually enjoying him. Conversely, while Cena was feuding over the US title, JBL had what I consider an awesome heel run as the WWE Champion. WWE built Cena up as the hero while they built JBL up as the cowardly heel who survives every encounter, usually through luck. It was very similar to the Honky Tonk Man's IC run.
JBL's run and Cena's run developed separately over the year and culminated at the perfect moment, one year after Cena beat the Big Show for the US title. It ended the way it had to, with Cena winning the belt.
WWE definitely took their time and developed Cena the correct way. Now, a lot of what Cena did after he got the belt was complete crap, but there's no way he got the belt too soon. WWE patiently developed him in a way they haven't really duplicated since.