The best way to read this series:
Read chapters 1-19. Then quit.
Ratman is a hilarious series that was ahead of both Tiger and Bunny and MHA in depicting superheroes in a non-traditional fashion. The premise is a hero nerd is tricked into being a villain, and while he is on the other side of his heroes, he discovers that the superheroes aren't as virtuous as he thought they were. Add to the fact that his evil organization is utterly hilarious, and that he has to hide his villain identity from one of his closest female friends, this series ends up being a lot of fun.
The series at the beginning excelled with both comedy and with building an interesting world. Chapters 18 and 19 are the best of this series, and deal with personal growth, which you don't often see in a shonen manga.
I'm not really sure what happened to the series after that, it almost seems different writing altogether. The series became bogged down with endless filler chapters, and the chapters that did pertain to the plot were just pointless action sequences. I'm currently at chapter 40, and struggling. I'm not sure I can or will finish the series.
Chapter 24, Shiki's intro, was the last good chapter of the series, but it's kind of out of order so I wouldn't recommend reading it unless you want to. I was just amazed by how they took a great character introduction and just threw away all the info they gave us.. but that became the standard for all following chapters.
If you're looking for a fun and creative superhero series, I would definitely recommend the first 19 chapters. But please, save yourself the struggle and quit there, you won't regret it.