It's quite hard to elaborate, but I'll just say that osu! (and its community) made me able to develop a more positive personality and attitude overall.
From mapping and modding, I have also learned that hard work, dedication and patience are truly needed to achieve what you want (to be a better mapper and modder, to get ranked maps, to have your maps liked by people, and many other things). Of course, there's still dashes of luck, but that's another matter.
In short, osu! made me a better person; that's all.
...Although I'm also becoming worse in time management because I'm getting so addicted (more like having too much business, though) to osu! to the point where I'll spend hours to be online in the game.
It's mixed.