Pretty damn enjoyable.
When I first started reading this series, I was a little skeptical. I mean, it was really popular, but I was afraid that, like some other series *cough Naruto
cough*, it wouldn't live up to the hype. And, in the beginning, although I was thoroughly amused by the plot, it just didn't do it for me. Then, things began to happen. It suddenly changed from a series about a plucky teenager on his many journeys into a tightly woven plot of intrigue, mystery, violence, science, and death. It was absolutely shocking, to be sure, but Arakawa actually managed to weave it in so well, and without sacrificing any of the good humor and characterization for angsty, emo characterization, that I thoroughly enjoyed the change, and loved the entire series. Still do.
The art... isn't much to look at, really. It's your standard shounen fare, with a lot of characters tending to have similar faces, EXCEPT that the composition of her fight scenes is excellent, and the detail that she puts into the facial expressions and emotions of characters is definitely evident.
Speaking of characters, lets talk about them. I'm going to say this now: almost no character that is EVER mentioned in this series is wasted. EVER. I won't spoil, but Arakawa manages to turn some of the most forgettable side characters (the ones that were in, oh, I don't know, three panels?) into full-on support characters with fully fledged personalities. And her main characters are amazing. Well characterized, they don't suffer from the usual Wangst that a lot of characters suffer from in other shounen. They react to situations in a way that seems ABSOLUTELY real, and as you read the series you begin to sympathize with them, and then when bad things happen it breaks your heart.
Overall, a very strong series, with an excellently written plot, lovable and well made characters, and decent art. *insert title here*