Luke Shaw is a decent player (not great - not good enough to play for Bayern, Real or Barca), though he may become great.
The issue I have is that he is mostly injured, unfit or fat.
Sometimes players' bodies are designed for pro football and I just get a niggling feeling that Luke Shaw may be sold in a few years for very little money or may walk out on a free transfer.
£30M for L.Shaw (most expensive teenager football player, who happens to be injured most of the time or unfit to play 90 minutes) is bad business, in my book. We got fleeced.
Conversely, I think Memphis (also a player bought for his potential) will become great in a few years time, so I have no issue with his price-tag.
At least with Fellaini, we got our money's worth out of him last season. He played most of the games and was an important player.