The bits faithful to the novel are really well done, and the new bits are fitting and thrilling enough not to offend my sensibilities. I particularly enjoyed the dynamic of Monsieur de Treville as police chief and the trio as brilliant and beloved but unruly officers, and hope there will be more of this. There's some gorgeous scenery porn, too, with lots of galloping through snowy forests.
Casting, from my point of view, was spot on, since Aramis was the hottest thing on screen at all times. Porthos and d'Artagnan also look just how they ought, though Athos seems a bit bland, and I liked Treville a lot. As for Peter Capaldi as Cardinal Richelieu...brrr.
It is quite hard not to picture them all as dogs, though. The King's hairstyle didn't help; at one point I said "Oh, you stupid spaniel!" to him out loud.