I LOVED Fallout 3, everything from the atmosphere to the gameplay worked wonders and really got me immersed into the experience. Exploring the ruined remains of a large city was really awesome and a fairly unique environment to explore, at the time anyway, there were a few problems I had with it but overall it will go down in history as one of my all time favorite games ever.
I picked up New Vegas day one, played it for around about ten hours or so then just...stopped. I think I know why though, for starters my character is too much of a blank slate. I don't usually have a problem with that, pretty much all of the Elder Scrolls start out with a blank slate character, but coming off of Fallout 3 which really gave me a good backstory and excellent motivations it seemed strange and weird, like I'd just jumped in and skipped the opening of the game.
Another is that I don't really get a 'post apocalypse' feel from the game, I mean the Mojave was just a desert, that's all, it's already a wasteland, there's no ruined buildings or destroyed railways to help me feel like there was a civilization before. It just didn't suck me in or make me feel important.
And good lord was the map small, I mean it seemed fucking miniscule in comparison to DC, I remember seeing a picture of the map online before buying the game and the first thought that came to my head was...well that looks small...I sunk over two hundred hours into my very first file of Fallout 3 and still hadn't seen everything, the majority of it yes but not all, while in New Vegas I took two steps out of that boring as shit town you start in and already discovered like half the map, or it at least felt that way. Not to mention I couldn't really enjoy exploring because FUCK CAZADORES!
Bugs are what ruined the experience for me honestly, I didn't get a good forty minutes into the game before I ran into a bug where my character's hand got all stretched out and weird, I looked like Mr. Salad Fingers, and it stayed that way for like a whole hour.
So yeah, loved Fallout three, couldn't stand New Vegas, I'm sure it would be better now if I gave it a try but I just can't really bring myself to do it, my initial impression was terrible.