Sora no Otoshimono is about a boy that meets an angel that dosen't know who she is or what's happening aka totally memoryloss. The story is very good and interessting, and it is very funny and ecchi (pervert), tho the ending is sad in a good way. If you like angels, ecchi things, funny things, guns etc etc, then you should watch this. There is so much I could say about this anime, but I dont want to because then I would only bust the story and ending.
And if you like the anime: Saikano you should watch this!
Well, the anime was good, the songs was good, the way they painted it was awesome. So go and watch it! :)