For some reason Del Ray decided that Shugo Chara! was much catchier than  the translation of My Guardian Characters! I agree that it is easier to  remember but I'm always amused at what titles get translated and what  don't. 
Shugo Chara! By Peach Pit (it's a manga-ka duo,  both draw and I can't recall if one does most of the writing or if that  it's a collaboration as well).
 Summary: Amu Hinamori appears to be a cool, calm, and collected  5th grader whose the envy of all the other girls. In reality she's  amazingly shy and is feeling quite frustrated with herself at the  moment. Enter here guardian characters, Ran, Miki, and Suu who are her  "would be selves," essentially her daydreams of being someone else made  real. She's not the only kid with these characters though and manages to  make friends with most and enemies with those who are being employed by  the evil organization Easter*. Evil organizations, a super-powered  student council, transformations and star crossed romances? Must be a  magical girl manga!
The Good: Figuring out who you are  plays a decent role (or at least a chapter) in any magical girl story  (or even kid/teen stories in general) so it's nice to make that the  whole premise of the story. And Amu is a likable character, she does  grow (and rather rapidly once she starts making friends) and doesn't do  anything too dumb over the course of the story. And I won't lie, SC is  my smutty guilty pleasure (yes it's a kids manga, Peach Pit just knows  how to cover both their fanbases).
The Bad: The story just  started getting really dumb towards the end. So, evil organization is  defeated eventually, yay! So we've got a few things to wrap up- huh, why  did we just start a new arc? Um okay, perhaps Peach Pit has bigger  ideas in mind, but wait it's all over! Alright, so that last arc did  absolutely nothing besides say "Amu is special!" and none of those  relationship issues got cleared up, huh, now there's going to be a  sequel series? And it turns out to just do a pair the spares  as the plot for each chapter? Well okay, THAT was really annoying and I  have to admit that I wasn't too satisfied with some of the wrap-up of  the evil organization arc. By the end of the story the plot started  dragging and then vanished entirely as the creators milked the last bit  for as much fluff and money as they could.
The Art: The  art is split between the two manga-ka and I think that one of them  definetly draws cuter characters than the other. The manga is full of  eye candy and, well, has more suggestive scenes than you'd expect a  story aimed at elementary school kids would. Not that I, or any of the  other fangirls, are complaining, and the outfits are very cosplay-able,  just be careful of all the ruffles.
Overall I liked the series in  the beginning, middle, and then the end left me pretty unsatisfied,  even the Shugo Chara Encore miniseries didn't make me feel much happier.  I've got the first five volumes (out of ten or so in Japan) and I'm  just not sure if I want to get the rest, I probably will but probably in  a few years when I'm a bit hazier on the ending.  
*I really have to wonder how many Japanese got that pun, heck it took me a while to notice it.