To really appreciate this series you need to know couple of things.

1. Author of this series originally drew this series in order to learn new manga drawing software he purchased. So you will see lot of silly and or in some cases stupid things, but as he went series got more and more popular and ended up being a one of the most popular anime/manga in the japan.

2. There is two version of manga one is made by original author and other one is illustrated by professional manga artist. So there is big gap between the two. You see, more serious Saitama was a creation of second artist and humorous, silly face of Saitama was created by original author, so both of them ended up mashing their talent and artistic sense to create one of the kind anime we see today.