Ahhhh, what can I say about this old friend of mine?
I first attempted to read Tsubasa in middle school and dropped it around volume 9-- I thought it was too boring. Years and a re-reading later, I have realized that this was NOT the right reason to dislike this series.
Many people say, "This is CLAMP", and that is justification enough for whatever compliments/complaints that they have about Tsubasa. I disagree. I have yet to discern any pattern among their many publications. The art in this manga is absolutely captivating and uniquely detailed; however, having read Man of Twenty Faces and Cardcaptor Sakura, I can say that their art has not already been award-deserving.
The concept behind this manga is fantastic. A cast of four characters hailing from three divergent worlds, Sakura, Syaoran, Fai D. Flourite, and Kurogane possess different goals but embark on the same journey to collect Sakura's "feathers", or her memories. They travel through myriad dimensions, encounter benevolent friends and nefarious villians. It starts off adventurously refreshing; by volume 15, however, you'll realize a sudden turn towards macabre truths and darkness.
Where is the issue, then? As others have mentioned, this series gets seriously confusing. I mean this with every ounce of integrity and emphasis I own. The explanation for the antagonist's actions are enough to make your head spin figuratively; the chronological warp made me physically dizzy. At the end, I wondered if it was truly my lack of intellect/deduction skills that caused my inability to process CLAMP's explanation. But having read other criticisms, I've concluded that this is more of an error on the mangakas' behalf. It's no fun to spell out every letter for your audience, and I understand that a bit of mind-warping can add some exciting zest. But the confusing elements within Tsubasa almost made the rationale seem... poorly written.
I have yet to read each and every of CLAMP's works, but with the exception of xxxHolic, none of the ones I've read thus far (Chobits, Legend of Chun Hyang, etc.) have been particularly memorable. I've found that despite the critical acclaim, CLAMP is not the best in storytelling.
Nonetheless... I fully recommend reading this manga. 100%. Yes, I wish the explanations were more convincing and less frustrating. But I see manga as more about the journey than the destination. I thoroughly enjoyed traveling to different dimensions with these four dynamic characters. The ending isn't half bad-- of course it's not a happily-ever-after conclusion, but given the gravity of the series, I didn't come to expect that. I will say that it felt a bit abrupt, so that's why my rating is an eight. If you ignore CLAMP's explanation and come up with your own, less-confusing rationale, you might just give this series a ten.