What's to say? It's one of the grand popular series -- when you do a Google image search on "sakura", 9 of the 20 pictures are of her (including one by some guy who made his own statue of Sakura naked...). It's also long, over 70 episodes with two major arcs.
For those who don't know the series (and if you don't: where the hell have you been?), it's about twelve-year old Kinomoto Sakura, who accidentally frees these animated magical spells, contained in cards. The spells run amok and it is up to Sakura to capture them and bring them under her control! The first arc of the series follows a 'card of the week'-format: every week there's something mysterious going on, it turns out to be a card and Sakura uses the cards she already possesses to capture this new card.
So far, it's nothing special really. Where it shines are the characters: there's cheerful Sakura, camera-wielding Tomoyo, grumpy Shaoran, competitive Mei-lin, friendly Yukito and the teasing brother. Because the series is so long, there is ample time for long-term character development, which is handled very well -- just as we expect from CLAMP! Another contributing factor to its popularity are the undertones (subtle and sometimes not-so-subtle) of homosexuality. Tomoyo has been described as "a lesbian in training", there's this thing between Yukito and Touya and between Shaoran and Yukito.
And then there's the humor. Sakura has a side-kick, the 'plush animal' Kerberos ('Kero-chan'), a small, super-deformed hellhound with a sweet tooth.
It's really hard to describe the whole series, we've watched it for over a very long period of time and it's a long series. I think the fact that CCS is the direct inspiration for my MagicCaptors campaign says enough. It's cute, it has good action scenes and good character development. It's an 8.