He had a very bright start to the season. Then he demonstrated why he's got a reputation of being inconsistent, because his performances were absolutely abysmal.
Personally, I think it's his footballing intelligence. His positioning is terrible, for instance. But he's gifted technically and physically - he's got a lot of good qualities. But he can play like a clumsy moron, out of position and unable to find a player with his passing. But when he's not playing like a clumsy moron, he's playing pretty great.
We're starving for consistency though and more often than not, we can't really rely on Can because we don't know which Can will turn up. And if he's not going to sign a new contract with us, I'm not too fussed about him leaving on a free. Sure, if he ever does click and start demonstrating what he can do on the pitch consistently, it would be annoying to lose him for no money. But at the same time, he's come in, fulfilled his contract, made decent progress... but at the same time hasn't made enough progress to where we're throwing money at him trying to keep him.
Like @Harry and @Inverted have said, that physique and his ability to make things happen in the midfield, and with his late movement in the box and vision in the final third... he could be a REALLY great player with some consistency. He's still got the potential to be as great as he can be. But I think we can afford to lose him on a free given his lack of consistency in our system.