What I liked most about this series would be the art. Looking back at it, CLAMP adds such a surreal feel to the manga. I have been captivated over and over, to the point where I stare at the pages and just "zone out" so to speak, blown away by it.
The concept of the story is also good. The idea of sending the characters from dimension to dimension was good and was well executed. It isn't always the same plot for each dimension. There are a wide variety of subplots in each world and some more serious than others. It's quite enjoyable reading it from start to end. But by constantly sending the main characters to different dimensions, we are never able to fully connect to the supporting characters and see their development. I didn't mind it much, but I'm sure others might.
Although the characters are constantly moving from place to place, CLAMP periodically teases us with a sense that there is a greater conspiracy than we were told before, and that turned out to be true in the end. While enjoying a variety of different stories in the different worlds, there is still a greater group of evil-doers that anchors the story down. It instills us in a mode of suspense to try and find out where the story is going.
Near the end of the series, due to the fact that the characters are able to travel through space AND time, CLAMP has woven quite a complex story. If you don't pay enough attention, you may end up getting lost and confused. I personally had to reread the chapters near the end multiple times in order to understand what was going on.
Overall, this manga was great! If you enjoy a manga that makes you think, but still makes you laugh, cry, and excited over the subplots and fight-scenes throughout its course, then I recommend this manga for you!