Sports manga in general (with a few notable exceptions) branch off into two main groups - the martial group and the teamwork group. The martial sports manga (featuring boxing, judo, etc.) usually sport characters with grit and guts struggling against one another. This type of sports manga also often suffers from overemphasis of the main character and weak side characters who only serve to show how awesome the lead is by comparison. The teamwork sports manga (featuring baseball, basketball, etc.) usually sport a large, well-developed cast. At the very least, the main team of a team sports manga will have a lot of fleshed-out players. The downside with teamwork sports manga is that it can sometimes feel like competition-lite, and it lacks the quantity of guts displayed in martial sports.
The American Football played in Eyeshield 21 is an interesting and ultimately successful combination of the two formulas, mixing the group dynamics of a genuine team with lots of hard-on action.
I have a lot of praise for the way the sport being played is used in ES21. Football is a large sport, requiring 11 players on each side, each with a different distinct role. As a result, you always have characters other than just the mains in play, being far from useless. 22 players, though, are a bit much to focus on at one time, and this manga acknowledges that, focusing only on small portions of the playing field at any given time. We get lots of different individual, small group, and large group confrontations throughout any given game, which makes up for the fact that a game may go on for 20-30 chapters. This effectively splits up screentime among a truly massive cast in a way which neither weakens the mains nor benches the non-mains. The cast is, in addition to being in the triple-digits, extremely diverse backstory-wise, ranging from a kicker who quit his team to help his father's business to a tall reciever who's good and popular, but failing to catch up to the true genius superstar of his own team. Odds are most, if not all, people will be able to find at least one character whose background they sympathize with.
And there are mindgames. Dear me, the mindgames. Possibly the best part of this manga is how the player confrontations are set up by a diabolical mastermind of a man, Hiruma Youichi, a gun nut with dirt on half the world's population. This guy comes up with the most outlandish trick plays which are usually a surprise, totally outlandish, and always fun to watch. What's more, they often don't work, giving an added thrill of uncertainty to each play.
The basic plot of Eyeshield itself is standard fare; weak, bullied kid (Sena Kobayakawa) with hidden talent gets forced into a sport and ends up liking it. His team gets stronger with him in it and goes on to compete at high levels. Just how high is a minor spoiler. Two things here. Firstly, you can expect the Devilbats to lose quite a bit, and not just in the introductory chapters to their eternal rival. Secondly, THERE IS NOTHING WRONG WITH THE BASIC FORMULA. Just because it's common doesn't mean it's a weak one. Coming-of-age stories have been selling since forever for a reason, and the basic formula needs only to be applied correctly to produce a decent piece. High-schoolers passionately fighting tooth-and-nail over one inch of turf isn't going to get old anytime soon.
Not that Eyeshield is just decent - it's consistently hilarious, thrilling, and a whole lot of fun. I would recommend trying it out to just about anyone, including the people who aren't normally predisposed towards manga. It's a shining example of what sports shonen, and shonen in general, can be when it tries.