Basic plot: Starts off as a typical Ken AkamatsuUnwanted HaremWITH MAGIC, but eventually shifts gears into a shonen type action-comedy. Negi is a ten-year-old mage prodigy who gets sent to Japan to teach an all-girl class. Plot focus eventually shifts to his quest to find his father, a famous war hero and one of the most powerful mages alive.
Content: Lots of Fan Service, Clothing Damage, Ecchi, and undetailed nudity. Nothing explicit though. There are some sexual jokes, but nothing really over the top (at least for Ken Akamatsu). Later chapters contain some bloody violence, but nothing really horrible.
The good:
Characterization: Even thought there are 40+ "major" characters, they are all unique, and Akamatsu does an excellent job of giving them all sufficient characterization without letting them steal the spotlight.
Artwork: The art is beautiful; everything is very well drawn. Yes, there is CGI Scenery Porn, but at worst it's merely unnecessary. For the most part, it makes the world feel more full and complete. The backgrounds are all incredibly detailed.
Plot: The primary story is an entertaining mix of action, drama, comedy, and romance, with some occasional parodies thrown in for good measure, all set in a Fantasy Kitchen Sink. Expect a lot of Lampshade Hanging. It does a good job of balancing the different aspects. The fights are suitably epic.
The bad:
The Myth Arc takes quite a while to get going. It doesn't really appear at all until the 3rd volume, and it takes a few more volumes to build momentum. It's well worth the wait though.
The Fan Service occasionally gets in the way of plot, although this can be at least partially blamed on Executive Meddling. Chapter 235 is a prime example of this.
Overall: Negima is probably one of the best mangas currently running. It's thoroughly entertaining, and maintains a unique mix of genres. It has it's flaws, but they don't have a serious effect on the quality. I highly recommend it.
One of the better series out there. (Manga only)
This is one of my favorite series ever.