W.I.T.C.H. is a brilliant show and I highly recommend it.
First of all there are the main characters, Will, Irma, Taranee, Cornelia and Hay Lin (hence W.I.T.C.H.) are interesting and likable, each with a distinct personality that works well with the others.
Another thing I like is that the characters are fallible. Too often in shows like this, the characters get their powers and instantly work like clockwork. In W.I.T.C.H., the Guardians can and do make mistakes. We get to see how they practise with their abilities to get better at them. Additionally, it's very good that when they transform into Guardians, they do actually transform, in that they become slightly older and sprout wings (compare that to shows like "Power Rangers" where all that happens is they get a change of clothes.)All these help lift it above most other shows.
The episodes are good, with a fine balance of humour and drama. What I enjoy is that unlike say, "Teen Titans", the show takes itself seriously, even when it's humorous (plus it doesn't have the ridiculous 'exagger-mation' or whatever it's called). That is, it's funny without being silly. The animation and artwork are also very good.