I think he has been an average signing so far. I think the vast majority of our defensive improvement has been due to Bissaka, who has already made as many tackles this season as Young made throughout the entirety of last season. Bissaka (47 tackles this season) has also almost reached the same number of tackles as Shaw, who was our top tackler last season with 55.
Bissaka is, again, positioned third in the total number of tackles made in the Premier League this season. Bissaka has made an enormous difference, which would inevitably have made the centre-backs' job easier. He can practically defend the right on his own.
In comparison, Maguire's stats have not been massively different to Smalling's stats. Maguire cost £80m. The improvement I have seen in central defence is minimal, especially when considering such a massive fee. You would expect far more for that kind of money. Defensively, he is no better than Smalling.
Maguire is better on the ball, but that is completely pointless when you haven't got any midfielders, aside from Pogba, who can actually pass.
He is a good player, but considering the massive fee, the neglect of the midfield in favour of him and that he is not a massive upgrade, I would say his signing has been an average one so far.