Far from the stale, stereotyped mass of catchphrases and kung fu the staff review portrays it as, Xiaolin Showdown is a fun, clever, heartfelt, entertaining show, great for kids and teenagers alike. Sure, there isn't much in the way of an overarching plot, but one of the most beautiful things about Xiaolin Showdown is that it doesn't really NEED a complex, nuanced plot to stand on its own as a great show. The characters grow and change over time, despite the impression you may get from a cursory glance at the show, and if you're really hungry for a good plot, it picks up halfway through the second season in a wonderful way.
One of the show's greatest strengths, for me, is the characters. I've no idea what show the reviewer was watching, with these "remarkably undeveloped" characters, because it certainly was not Xiaolin Showdown. The characters all start off as flat caricatures, it's true. However, as the show progresses, they grow and change and develop far beyond the stereotypes they embody. Their cliches are not meant to embody their personalities, but rather to contrast them, as demonstrated with folky cowboy Clay and his penchant for action figures, trivia, and clever problem-solving. To get hung up on the cliches and see the characters only as the stereotypes they embody is to miss out on a large portion of the show.
Not to say that there's a lot of heavy character development, because there is not. This is a lighthearted comedy-action show, with more focus on funny gags and well-choreographed fight scenes than on plot and character. However, the character development and plot are still there, crafted with such subtlety as to blend into the background. Compare the motley cluster of preteens in the first episode to the confident team of seasoned warriors in the last, and it becomes glaringly obvious just how much the Xiaolin Monks have grown, both as warriors and as people.
The villains of this show bear mentioning as well; there's not so much as a whisper of them in the original review, which is a shame! The main villain—at least, the most visible out of all the recurring villains—is Jack Spicer, a self-proclaimed "evil boy genius" with an army of robots at his beck and call. He starts out as the biggest threat the monks have to face, but as more villains make their way into the picture—including Wuya, an ancient witch trapped in a puzzle box 1500 years ago by the great Grand Master Dashi, and Chase Young, an immortal warlord with a taste for dragon soup—it becomes clear that Jack Spicer is the least of their worries.
The show carries a number of strong messages, which it conveys without getting too preachy. Lessons about teamwork, about family, about how making the right decision is not the same as making the easy decision, about how making mistakes is okay and even expected - all these and more, woven into the storytelling in a way that is visible but not ostentatious. The characters make mistakes—lots of mistakes! Huge mistakes!—and don't win every battle, but they come out so much stronger for it. That, I think, is a message that more kids need to hear. Not just that "winning isn't everything," but that losing is a part of life, a part of growing up, and sometimes you can gain more from a loss than you ever could from a win.
As far as content goes, this show is suitable for just about anyone who enjoys fast-paced action, lovable characters, and clever humor. There are copious amounts of cartoon violence, as the main premise of the show is centered around fighting, so parents concerned about violence may want to steer clear of this show. It's not imitable or gratuitous; mostly slapstick, and some punching and kicking. There are some dark/scary scenes in certain episodes that very young children may be frightened by. Personally, I've watched the show with a number of children ranging in age from 5-15, boys and girls, and they all enjoyed it.
Xiaolin Showdown has been off the air for 7 years now, but you can easily find all the episodes free to watch on Youtube. The first season is available for download in HD from iTunes; I highly recommend purchasing it in order to support the show (and the episodes are absolutely GORGEOUS in HD!). There's also a sequel series, Xiaolin Chronicles, coming out sometime in late 2013-early 2014 (a release date has yet to be announced).
As a passionate fan of the show, I urge any parents who see this review to give the show a chance before writing it off as another flat, stale kid's action show. It's so much more than that, and it deserves so much more than this site's review gave it.