Honestly ? It's far to be the best CLAMP work. This kind of story has already been used and there's nothing original in here. 20 Masks outfit reminds me too much of Kaito Kid which is also a bad point and well, I think the relationship between the two maincharacters is simply unbelievable. It just seems weird... The art isn't really the best either.
That apart you have the typical characters : one chara who ALWAYS want to catch the thief, the thief always being able to escape; and overall each chapter seems the same, there's not the slightest surprise, so I do not get the
feeling anything is happening at all. In other words : boring. Kaito Kid, Kamikaze Kaitou Jeanne, Cat's Eyes and DN Angel are far better mangas of this genre.
For those who are lazy to read : not worth reading.