CLAMP have, as per usual, created a very complex story with huge psychological impact on both the characters and the readers. The characters are great and they each get their fair share of development and angst throughout the series.
Tsubasa may seem like a dimension hopping fun house at the beginning but gets darker and edgier about halfway through and CLAMP, once again, make it work.
I'll keep this short for my own sake because otherwise I would ramble on for hours. Every character is believable, has his/her own strengths and weaknesses and a purpose that is unique to all of them but that come together nicely in the end.
The art is gorgeous and the story captivating with it's mind screws all over the place though also very confusing. Even CLAMP got confused in the end.
I recommend this manga to pretty much anyone but advice you to not take the anime as a serious adaption because it is not.
Anime is actually frowned upon by CLAMP and they had season 3 cancelled. So stick to the manga (except the OVA's) and enjoy.