Got this last week or so.
It's Rockstar's latest game; very reminiscent of their Grand Theft Auto series, but set in the American Wild West, around the turn of the century (ca. 1900). It's more story-driven than the previous games; less humour (but still tongue-in-cheek where appropriate) but with plenty of gunslinging action. I'm loving it so far.
Graphically, it uses only a slightly re-skinned and touched up version of the GTAIV engine, but I must say it looks excellent nonetheless.
Definitely recommended, though, if you own a PS3 as well as a 360, the 360 version will probably be the better version visually (owing to the fact that Rockstar didn't bother to optimise the graphics engine much from GTAIV - which used a proprietary engine better suited for PC-like architectures).