Saddle up partner! The most fun I've had in the west that I can remember.
WHAT'S GOOD:
The style - I know there's at least one other western game out there for the PS2, but that's pretty few and far between compared to other genres. So it's nice to have a new kind of atmosphere to find yourself in, and I think they pull off a gritty wild west theme very well. I doubt it being historically accurate, but it's fun.
Stuff to do - There's lots of types of activates to perform in Gun. Rescue missions, bounty hunter missions, item deliveries, gold mining, and more.
Voice Acting - There are some excellent actors selected for the voices.
Mild RPG Elements - I'm not big on full-blown-RPGs, but I don't mind when there's RPG elements in a game, like this one. You can upgrade certain characteristics in two ways. The first is by completing missions (both the main storyline missions, and side missions). Based on the type of mission it was, pre-determined characteristics will be upgraded (e.g. the ability to ride horses, marksmanship, etc). The second way is to buy tools to upgrade characteristics and weapons of your choice (e.g. the speed you reload certain weapons, how strong your weapons are, etc).
WHAT COULD HAVE BEEN BETTER:
Quick Draw Control Scheme - When I'm in 3rd person view, I like to have my horizontal control inverted. When I'm in a 1st person view, I don't. In Gun, you are usually in a 3rd person POV, but Quick Draw mode puts in in 1st person, however it doesn't have a separate H-Control setting, which mixes me up.
A few bugs: Not major things, but little things do happen. For example, every now and then I'll ride a horse out to a side mission, get off the horse, complete the mission, and then my horse has disappeared so I've got to walk back to town. Another time I had to repeat a side mission because I did a part of it too quickly (out of order) and that confused the game preventing me from finishing the mission. These types of things didn't happen a lot, but when it did it was kind of a pain.
Occasional Deja Vu: Although there's lots of mission types to complete, each type of mission doesn't differentiate itself from each occurrence. That is to say, all the bounty hunts more-or-less feel the same. As do the pony express missions, as do the animal hunts, etc. It's a similar experience for the main story missions as well, although not quite as repetitive. Luckily, the missions are still fun, so it's not so bad to feel like you're repeating them.
FINAL WORD:
There's other 3rd person action games that out-rank this one, but very few will deliver the experience that Gun does. If the idea of laying down your own law in the wild west sounds at all fun to you, then I would recommend it in a heart-beat.