I think we are the ones that assumed he would be a Bale replacement, and let's face it that is quite a difficult player to replace and perhaps an unfair expectation!
He is not responsible for the transfer fee, but I would argue that we have got as much out of him as Chelsea have Willian, who was the one we were going to get. He's not our best player, but he is by no means the worst, and I think that in order to upgrade we would have to pay a much more significant sum of money than we have, or are prepared to pay. Players like Bolasie who have much less end product are going for £25m!
I think that he is on the whole underrated, and under performing. He has performed well (scored 11 goals and assisted 9) last season with just under half of those goals coming in the EPL. I think he should be scoring 10 in the league and assisting the same and to be fair he is not far off that. I'm not ready to write him off as I think there is the potential there, but it is the mentality that I like. He works hard, is not a prima donna, and seems to be a good player to have around the squad (his comments in the league run in last season I thought were good to have).