When I first heard of Eyeshield 21 from a family member, I was lacking interest. A manga about (American) football? Eh, not really my sport. If it was Baseball, I would've looked at it right away. I read through a few pages, maybe a full chapter, and then just left it.
A while passed, and I ran out of other manga to read. I decided, "Hey, might as well try Eyeshield 21". The first few chapters were not all that interesting, but the pace quickly picked up and I was hooked. The art style remains iffy for about 50 chapters or so but it gets a LOT more stable once the main arc starts up and is brilliant the rest of the way through.
The exact appeal of the manga is difficult to point out. A part of it is Hiruma Youichi, the best manga character ever conceived of. I'll let a quote speak for me:
''"As it is right now, it's over. Turning the tables is already nothing but physically impossible. At least, in memory of the fact that we managed to come to the christmas bowl, even if we lose in the end, it'd be great if we could at least get a touchdown. At least let's end this by destroying Yamato's absolute prediction of keeping us from scoring a single point. THERE'S NO ****ING TRASHES THAT WOULD THINK LIKE THAT IN THE DEIMON DEVILBATS, RIGHT!!?"
"OF COURSE, KORAAAA!!!"''
— Hiruma Youichi and teammates,
And of course, that's only a tiny bit of his awesomeness which includes being able to blackmail any person on the planet, his insane plans and his personality.
Another greatly enjoyable aspect of the story is its over the top humour. Sena's extremely frail body being the butt of many jokes, the hah-hah brothers, Monta and his resemblances to monkeys, etc, etc. The series has plenty of moments of comedy that will get you laughing more than School Rumble.
Of course, since it's a shounen series, it has to have drama, but it's really well done. There's not much more to say in praise other than it's just excellent.
As someone whose never really liked sports stories, Eyeshield 21 was a definite break from the mold. You have to read it through to figure out how great it is but regret it you won't.