The thing is, when we still had Fergie as our manager, Fellaini was constantly linked to us and almost unanimously, people agreed he would be a good signing for us, bar a few. They (and me) also agreed that he was a good player who was good enough to make the step up here. Even last summer people still agreed he was a good player, but the consensus was that he wasn't the top quality player that we needed to sort out our midfield, but as long as we did also get a top midfielder in then it'd be fine, which we sadly didn't.
People now act like he's the worst player on the planet and a shambles etc, but it's just not true at all. He's a good player, he's just in his first season, with a few injuries, in a team that has capitulated to unprecedented levels.
I think the biggest reason for the mass criticism of Fellaini is actually mostly due to the fact that it was Moyes who signed him, and people see him and his ridiculous transfer fee as just another way to parade how shit Moyes has been here, seeing as he was his only signing and he came from the club he used to manage. He's an easy example to use of Moyes' failings here so far.
I think he's good enough for us, but I can see why people will disagree as he's been shite so far, but if he's here next season (very likely) I'm pretty sure we'll see a much better player than we have seen this season.