I actually rate him relatively highly. He's far from what he was originally hyped up to be, but I don't think he had a bad season at all, managed to play regularly in an (albeit poor) Arsenal midfield and whenever I watched them he seemed one of the brighter players on display. When we beat them he was by far their most dangerous players on the ball, and unlike some other Arsenal players he at least seemed like he was willing to give everything for the shirt. Injury wise he seemed to be alright last season, although a player with his record could break down at any minute, so I understand why Southgate left him out of the England squad.
I don't think he developed very well as a midfielder mentally though. A lot of the time it seems like he attempts to take players on unnecessarily, when a pass is the better option. Whilst it's nice to see a positive attitude from an English CM, he often beats his man then passes it sideways anyway, so it's often a case of him really not achieving much but looking good doing it. West Ham is a good level for him, he will play regularly and if he can prove that the injury issues are behind him (doubtful) then I'm sure Southgate will give him chances, it's not like we're stacked in that position. I do think people actually underrate him and say he's terrible because of the early hype and because he's a bit of a nob, he genuinely did look pretty decent for Arsenal last season.