I don't know how to explain it, but it is, and it isn't. It is in the sense that technically, it is a GTA in the wildwest, but at the same time, I feel that description sells it short.
There is a level of quality and polish here that sets it apart from previous GTA style Rockstar games. There is also a real sense of identity here. You can tell they really put their soul into creating something very unique.
When I first started the game last night, the first thing I was really impressed by, were the visuals, which are great. I also love the edgy art style, which is more Tombstone than Brisco County Jr. For example, the main character has a face that only a mother could love. Even the main female lead looks a bit weathered.
I only played a few hours or so, so I am not sure if this will start to feel like a typical rockstar game as it progresses. I remember GTAIV had a grand first few hours before it went back to the standard.
I've been pretty impressed with the shooting so far though.
I had been wondering recently what it was about Rockstar games that made them feel dated so very quickly after release, more so than other games. My conclusion was that Rockstar games have always been highly impressive in certain areas (like alive open worlds), and kinda frustrating and shitty in others (like the actual gun play, the lack of save points), even upon release.
So far the good news is that RDR seems to have very good shooting mechanics, that are really enjoyable. The horse riding is just awesome as well.