Judge him as a player, judge him based on what you think he's capable of, judge him based on his previous performances and what you perceive to be his potential. Just him based on what he does on the pitch and only what he does on the pitch.
Transfer fees have become more and more detached from the real world and are so wildly variable that attempting to judge a player based on some intangible concept of an "£Xm player" will never make sense. Is Pogba performing 60% better than John Stones? Is he 50% better than Oscar in China? Is he twice as good as Sterling? None of those comparisons make sense.
Rival fans will take the piss when an expensive transfer isn't an instant success, but I would expect fans of the same club to be able to rationally discuss performances based on more than that.
My mentioning marketability is in no way a defense of his recent performance level. He's not been good enough. Full stop. Not "not good enough for a £90m player", just not good enough.
The marketing issue is, however, entirely relevant to why he cost so much money. Adidas will be happier paying us big money when they can show him in United adverts, as will every other company we have sponsorship deals with, he drives internet traffic and makes advertising have more impact on target demographics. That means it forms a part of the cost/benefit analysis when out CEO decides how much he's willing to pay, along with X number of other things, which is why deriding a player based on a transfer fee is pointless.
I'm not saying he's playing well at the moment. He's not (although he's not as bad as some are making out and others have been worse). I'm just saying leave the money out of it.
And now I really am done.