I can't believe I couldn't find anything about this game on here.
I bought it used last night for $25, and it's worth twice that, easily. I also bought Red Dead Revolver (again, used, $15) because I'd heard nothing but good things about it, and played it first for about an hour and a half. Then I wanted to check out GUN. After about 20 minutes of playing I thought about taking Red Dead Revolver back, not because it's a bad game, but because I'm not going to play it much until I'm done with this one. If you don't know, GUN is also an old west style game, but I like the controls and weapons much better. It's also a bit more graphic/gory, for instance if your enemy if hurt or dying you can SCALP THEM! That is too cool! You can do Pony Express missions, catch people on Wanted Posters (dead or alive), play poker (though I haven't got to do that yet)... there's just plenty to do to keep you busy outside of the story missions, which is great because I'd hate to give this a good review and then beat it in two days. I don't think that's going to happen though because of all the extra stuff.
Anyone else playing? If not, I can't recommend this game enough, especially if you can find it used and relatively cheap.