It's already completed and the story, arcs and characters are way better portrayed there.
The anime adaptation, however, was only a poorly executed atempt to advertise the manga. But don't get me wrong, it is, in fact, not a complete trainwreck. Each episode mirros the original source really well and the humor is pretty much the same. The voice acting is decent to and Inori Minase as (best girl) Nagisa is, as usual, masterfully well acted from her part.
The weak points of this series is the fact that it's only 13 episodes long, which barely covers all of the character development the manga has in its totality, not to mention that some characters that are of great importance in the manga don't even appear in some of the episodes. So yeah, if you manage to finish watching them all, you'll feel that something was missing and that something just didn't feel right. And that's exactly why: it only showed a portion of the whole thing that comes after the arc that was covered in the anime.
If you're a fan of romantic comedies and monster girls, this series is definitely for you, but only watch the anime as an introduction of the whole thing that the manga of Jitsu wa Watashi wa is, because it is absolutely incredible and I cannot recommend it enough.
