I'm not surprised that Walsh and Koeman can look at Niang's immense talent and think they can be the ones to fix him and realize his vast potential. However, in my experience that almost never works out. Players with poor attitudes very rarely change their stripes. At £16 million, I don't think he'll be a horrible signing but I also don't think he'll ever become a star.
All that being said, we can use a winger with his brand of speed, strength and skill. I don't think his value will go down in the next few years so I'm not too concerned. My only issue will be if this transfer prevents us from going after a better player. At the very least, he'll provide us with something different.