I thought it was well done, because no one really saw it coming. I mean look in the news section of this site, there was the interview with Cena saying he wouldn't even be back till after Mania cause he was doing a movie next. And BAM, not only is Cena back at the Rumble, but he wins it and is going to Mania. With how the internet is, how often is the WWE able to shock someone, especially with the Rumble Match itself. Usually there are 2 to 3 guys that it will most likely be and always is. But not this year.
Think about this too. Every Royal Rumble Winner has always been a pre-announced participant. Because a suprise entrant wouldn't have the proper build to go into Wrestlemania as the #1 contender. Like back in 2001 when the Big Show was the suprise entrant. It was a great suprise to see him back, but you knew he wasn't going to win...and its not like he hasn't been in Main Events before so why not, but its not good.
Cena has the background with both the WWE & World Champions to be the perfect person to come out of know where and still get a good run to Mania. He's the only one who could have been a suprise entrant and win.