There were some things that I liked about the art and some that I didn't. I'll go with the pros first: good detail, realistic scenery, smooth animation of moving objects. As for the cons: characters look awkward at times, annoying reddish shades on knees, exaggerated lips. From this, it's easy to tell that I liked the artwork and animations for anything not human. The excessive use of sparkles didn't work for me most of the time as well. The OP and ED sequences were interesting and unique.
Characters (8/10)
If you were to take this from a teenager's point of view and a young adult's point of view, the score might be different. If you like characters that develop and change their thought patterns and ideals as the story continues, then you may rate this higher than me. However, since I dislike long and detailed soliloquies, I didn't pay much attention to them, thus, the characters were less developed in my point of view. Nonetheless, it is obvious that the main protagonist, Niwa Makoto, had his views of different aspects of life change a lot during these 12 episodes. Whether these changes are good or bad, it was nice to see this development. As for the other characters, they were interesting but not memorable. Although I must say that Touwa Erio easily takes the award for most moe character of the Spring 2011 season. Hanazawa Kana making an appearance as herself was a nice surprise.
Sound (7/10)
Like many others, the first time I heard the OP, I was extremely surprised. However, after listening to it multiple times, I found it quite catchy and addicting. The ED, on the other hand, didn't quite fit my tastes. As for the BGM and voice acting, they weren't particularly outstanding. Only the voice of Hanazawa Kana was recognizable for me but maybe that's just because I haven't watched much anime. Other than that, the sound didn't really play a huge role.
Story (7/10)
Honestly, this anime could have ended when Erio realized that she isn't an alien. Dragging it on for that many episodes decreased my rating. For the last half of the series, there wasn't much of a "story"; just Makoto rambling on about life and whatnot. Maybe I not paying attention to any character who talked for more than a minute straight is what made me believe that a story line was virtually nonexistent. If split into halves, the first half would be a 9/10 for me while the second half falls to a 5/10. Average out the two and you get 7/10.
Overall (7/10)
I didn't really expect anything from Denpa Onna to Seishun Otoko. Maybe the appearance of aliens would've enhanced my experience since they talk about them in almost all the episodes. The only reason why I stuck with it was because of Erio. Yes, during every episode, I watched in hopes of Erio showing up and giving me my episodic dose of moe. Not a disappointing series but could have been much better. Maybe Episode 13 may influence me to edit my review. Who knows? But for now, the score of 7/10 stands.
