Edge from Hello Games' Sean Murray, his studio's recent PSN release, Joe Danger, only had a 10 percent completion rate.
"Only about 10 percent of people who bought Joe Danger have completed it (their average play time is well over 20 hours)," he relayed. "Most journalists didn't get past the halfway mark. Was it a wasted effort to add those last few levels that most people don't see though? Is it valueless?"
It's a hot topic in the games community -- how long does a game need to be? How much of a game's value is specifically tied to how long it takes to complete it? Then you add in Trophies and Achievements and you've got a whole new layer complicating the argument. It's a good read, so hit up this link and give Murray's essay a gander.