Comparing books to on screen representations is always tricky; what you see on the page isn't going to be the same as others may. The Musketeers have appeared in so many guises, from Charlie Sheen and Kiefer Sutherland camping it up in 1993 (between this and Batman and Robin, it's amazing Chris O'Donnell has continued to do so well), through to the kiddy cartoon 'Dogtanian and the Three Muskehounds'. Do a search for a picture of Porthos, and you'll likely find Captain Archers Beagle from Enterprise. They have become a pastiche of themselves, more known for their catch phrase as their actual back stories.
None have really got as close to the original books as this series.
Aramis may be a former priest, who seems able to charm any lady he comes into contact with, but neither of those qualities are over played. Athos is a dark, brooding, level headed father figure in the group. His back story with Milady de Winter becomes apparent quite quickly, and is sure to continue to be a thorn in the side of the Musketeers adventures. Porthos becomes much more real in this series. Still larger than life, still living life with violence and a smile, but with a much broader history and reason for being the way he is. They've fought in wars in the name of the King, and come out with not only the scars but the camaraderie that goes with it. Captain Treville does a great job in keeping them in line.
And D'Artagnan is the country bumpkin. Ruled by his passions, his skill as a Musketeer are evident, but he still need tempering. He has great great chemistry with Constance, who is friend, mummy and potential love interest to him, but like Athos/Milady, that journey will take a while to reach fruition.
This is definitely the best Cardinal. Yes, he's evil. Yes, he plots and schemes to destroy the Musketeers. And, yes, his motivations are driven by more..fiscal..desires. Yet, he's also a lot more believable and watchable for doing so. Not just some one dimensional monster, with a single plan, he even helps the Musketeers at times (for his own benefits, of course). The Musketeers may not like him, but nor can you see them running him through, like they do to many of their other foes.
The sets and costumes are great; from the Musketeers understated leather armour, to the Cardinals pitch black flowing cloak that make him look like a Dr Who baddie (yes, the irony). From the Royal palaces with their sumptuous ceilings, to the hustle of the newly redesigned Paris. It flows. The quality may be film set clean (and there may be the occasional Hollywood explosion), but it all adds to the overall rip roaring adventure, that fits perfectly into the time slot allowed, while the characters and their interplay pulls the stories along at a brisk pace.