The cast is overloaded with great actors who genuinely seem to fit in a Western. Ed Harris and Viggo Mortensen are great as the pair of tough-as-nails lawmen who stroll into town. I like their interplay with one another and they seem to be in control from the start. Jeremy Irons is a delight as the villain. He has a number of evil moments, but even just his presence comes across as dangerous.
Most of my problems with Appaloosa revolve around Renee Zellweger. For starters she does not feel authentic to the era, she's a modern character shoved in attire from the old west. Her character is such a frustration in the story as well. Some of that is intended but it makes so many of the events irritating or unsatisfying. The conclusion of her story in the film made me chuckle, it was so stupid.
The only other struggle I had with Appaloosa was the passage of time. They didn't make it clear when time had jumped ahead. There were just these awkward moments when it seemed like things had progressed so far between scenes and I thought "Oh, this must be months later or something." Simply making more use of the Viggo Mortensen voiceover they used in the beginning and end would have solved this easily enough.
I still enjoyed the movie, though. It has some great showdowns that have that tension and excitement you want in any Western. It also has a few twists and turns in the plot I didn't expect. Plus all that solid acting was just a treat. You can see I had some complaints with Appaloosa, but my general investment in the characters and interest in the story made it enjoyable, even if I won't necessarily seek it out again.