Some insights from a Leverkusen fan who has watched him more or less week in week out since he arrived as a replacement for Adler. In the beginning, he was totally amazing. Was a direct improvement to Adler who himself was considered on par with Neuer for quite a while (and briefly looked to win the spot as Germany's number one keeper). However, as many posters in here already explained, he didn't really develop since then. Especially the last two years were characterized not only by stagnation but also many mistakes. He recently seemed to have problems with his nerves when he played on the big stage (like the national team). While he has brillant reflexes and is very good in one on ones, he ist not that great in controlling the box and his passing and ball control is very weak for a modern keeper. Probably the biggest weakness he has and it is a pity that he never worked on it.
However, he still has all the talent in the world. I feel like this step comes waaayy too late because he would've needed to get out of his comfort zone earlier in order to develop. As mentioned before, he was on par with Ter Stegen and looked like one of the biggest goal keeper prospects in the world a few years ago.
I think currently, there are few keepers who are truely world class: Neuer, De Gea, Courtois, Ter Stegen and Oblak. Additionally there's Donnaruma as a future candidate and Ederson as a brillant keeper in a specific system. That's seven players in total and typically Barca, Real, PSG, Bayern, United and City insist on having them in their teams (and probably will, sooner or later). Not much left for the Arsenals, Chelseas, Juves, Milans, Inters, Atleticos and so on theoretically. The tier below that is pretty broad and I think Leno belongs in there, just like many, many others so I think it is generally a good signing for Arsenal. And unlike many other goalkeepers in this tier he still has much potential for development, especially once he matures a little bit which he undoubtedly has to do now he's out of his comfort zone.
Besides that, I wish him good luck and am not too sad that he finally left us. Which says something about his development at Leverkusen, I guess.