Fantastic piece of work made by two incredible people.
Story (9) - OPM breaks the typical hero manga pattern even though it's all about heroes. It isn't just about saving the world while saving the girl and beating the poop-head that wants to kill everybody. No. It's about how funny and exciting it could be to save the world as a hobby. Everything's over the top and does't shy away from anything. There's not much plot, but definitely has the potential to develop one. Overall, it's the type of story that's just honestly entertaining and keeps you at the edge of your seat. Excellent.
Art (10) - Murata Yusuke-san was the whole reason I got into the manga. He's a phenomenal artist in every way - color, lines, details, variety, technique, story-telling... I could keep listing them for days if I have to. His character designs are on a whole nother level; each and every character has a look that's great on their own right. Murata also has a way of making big things even bigger, and epic things seem god-like - perfect for something like OPM.
Character (8) - Saitama is not your typical hero. He's unmotivated and extremely strong from the start. He's not extremely righteous but he's admirable enough that's he's hero-worthy. You can look up to him, make fun of him, and identify with him at the same time even though he's an odd character on so many levels. On the other hand, the other characters seem to fit with your typical hero-story themes: the valiant, the wicked, the haughty, the modest...etc. Seeing their interplay with Saitama is a great sight. Overall, though not very complex, the characters are undeniably interesting.
One-Punch Man is definitely something to watch out for in the future. Even though it's only been out for a couple of years, it's already such a powerhouse; it has great potential so I'm expecting a lot. A must read of shounen/action/hero-manga enthusiasts.