I'm only 3 hours in, but I'm really loving this game. To me at least, it feels a lot more like "Fallout" than Fallout 3 did. I liked Fallout 3 a lot, but something about the atmosphere, characters, and writing feel much more like the old games to me. I'm sure the setting has a lot to do with this, as well.
I haven't experienced any bugs or performance issues (Core 2 Quad, 4GB DDR2, Radeon 5870, Windows 7 64 bit), but I'm making sure I keep multiple saves just in case.
edit: I'm also playing in hardcore mode, and while I haven't much noticed the differences yet, I already enjoy it more just knowing that I have to keep track of food, water, and sleep. It makes my scavenging that much more meaningful!