Obviously he's been incredible. However, I still feel that the Celtic fans opinion hasn't come from thin air either. No offense to the SPL but it's hardly the Premier league in terms of quality.
At this point my one reservation is that Johnstone is a bit of a one-trick pony who cuts in and shoots but can't really cross. Aside from Arjen Robben who was a master at this, it's not really a viable strategy for a winger as it's too predictable/easy to defend if it's your only asset.
As said, I'm really happy with him so far and he's been a big upgrade on Sarmiento. But this is my only worry. I guess time will tell by the end of the season and hopefully it's not the case.