I enjoyed Alatriste; it's not your typical fast-paced Hollywood action flick (if you go see it expecting something of the sort, you'll probably be disappointed and bored) and the plot is not too clearly defined, but it has an excellent cinematography and costume design that recreate Velazquez's Spain, and most of the actors are very good. Mortensen is an awesome Capitán Alatriste despite his slight accent. You can see he took this role very seriously. He fits perfectly into the roguish, ruthless but noble character's skin. The sword fights are nicely choreographed. The atmosphere of seventeen-century Spain and the historical context are superbly recreated. Actually, I find there are interesting parallels between the decay of the Spanish Empire and present day United States, between the "tercios" and the US Marines.
I recommend this film highly to anybody who's interested in period films, or who likes Mortensen as an actor.