It's pretty good, not that bad, and only a little ugly.
The story is really an accessory, and completely linear. You play primarily as Red, who feels like an amalgam of several of Clint Eastwood's characters from the '60s through the '90s: Red's family has been killed (see The Outlaw Josey Wales), he's quiet and squinty (see Leone's Spaghetti Western Trilogy), corruption runs rampant (see Pale Rider or Unforgiven) and there's a cocky Englishman (see Unforgiven again). None of this really matters because the story's just a thin thread to hold the missions together.
Combat ranges from really fun to really frustrating, but not frustrating enough to completely ruin the experience. The shooting mechanics are decent and incorporate several types of weapon, from hand guns to rifles. Movement, on the other hand, can get quite clunky at times (a train-hopping mission springs immediately to mind). Then there's dueling, which can be hit or miss (quite literally) in terms of difficulty. Dueling involves a combination of slo-mo, working the analog stick, and firing in order to take out your opponent before he takes you out. While it's, for lack of a better word, a neat little gimmick... it feels like exactly that: a gimmick. There's not a lot of consistency in terms of where to hit an opponent in order to take him down and it can result in a lot of frustration and retries. The inconvenience of some save points adds to this at times, as well.
All in all, Red Dead Revolver's a decent game, especially given its price tag now that the PS2 is going kaput. It's different from most action games available, but merely being different doesn't make it great. If you're a fan of third-person shooters, the Wild West, riding livestock, or Clint Eastwood before he started training girls to box, check it out.