I find the series alright, though vastly overrated by fanboys. As Wolf said, it loses a lot of quality when Saitama has to finish a fight with more than one punch, especially when that's the main joke of the series. The way fanboys inflate his power is even more ridiculous than those of most other series, such as viewing Saitama VS Goku or even VS Superman as not being too far a stretch, despite him having to take a planet level threat seriously.
The humour's alright when looking outside of the overused main joke, though nowhere near Gintama level. The art's alright too, though not as stunning as people are raving about. The only thing that caught me as being amazing was the opening, which is very catchy, which can be expected from something done by JAM Project.