I'll agree with you that Catherine Zeta-Jones is a very mediocre actress, and her displays as a supposedly "cougarish-like" enticing babe leave much to be desired, and that thus, conversely, her representation in Traffic is, as was to be expected, very poor.
I will also agree that Michael Douglas is not all that.
Finally, I will agree that the remainder of the cast (the Swimfan girl, who plays the junkie daughter, making use of perhaps one single facial expression throughout the entire fillum, as well as her multiple drug-ingesting classmates, and so on), with very few exceptions, is watchable at best.
However, you cannot intend to suggest that Benicio Del Toro's performance is nothing shy of pure genious in that picture.
To be frank, Benicio's performances tend to impress me wherever he is. Be it Fear And Loathing In Las Vegas, Snatch, Sin City, or 21 Grams. In any of those movies he leaves me flabbergasted, to say the least.
Nevertheless, it is not solely Benicio's astounding performance (which I believe earned him an oscar?) in Traffic, that makes the movie worthwhile. A few other elements (Don Cheadle, his partner, Dennis Quaid (very impressive display from him in this movie), even the drug dealer who's to be a witness in Zeta-Jones' trial plays a solid role, the corrupted Tijuana general - Salazar, the homosexual hitman Francisco, Manolo (Del Toro's closest friend), Manolo's girlfriend, the American general Douglas conferences with throughout, and one or two more) also add to a movie that, whilst it may have one or two moments that bore the eager eye (a tad slowish), is overall very well elaborated.