In my first playthrough of New Vegas, and at nearly the very beginning of the game itself, I cheated to give myself the necessary caps to enter the Vegas main city. I knew I was cheating, but I didn't realize that the cheating would in turn remove my ability to get any achievements, and so after that discovery, I promptly gave my character all the unique weapons and armors, endless supplies of special bullets, modified my stats and generally wasted the experience. I then set the game down, touched it once three months later, then didn't actually start it up again until a month after that.
The second time was where the game took off for me. The locations, the quests, even just the general feel and atmosphere of the game, made it a great experience and one I'm still going through (still haven't beat Old World Blues, and the Lonesome Road DLC Just dropped.) As with Fallout 3, the sheer size of the game world and all the different things you come across make just wandering around a game in its own right, and then there was the campaign. Those alone gave me around 100 hours played, and then there were the expansions.
The expansions are all amazing quality. Dead Money, Honest Hearts and Old World Blues haven't ceased to each blow me away. DM was a crazed, winding ride, with a payoff of more than I could ever possibly spend (I managed to take all 37 or so gold bars from the vault.) Meanwhile, where Dead Money had its intense, shadowy heist, Honest Hearts had spectacular scenery (Look up pictures of Zion National Park- they recreated it pretty faithfully,) and its human side made it a different, but beautiful experience.
After that is Old World Blues, which even despite my not being done with it, is the best among what are already three great DLCs. It's humor is exactly what I've been told to expect from Fallout, and so far the only game which beats it in making me laugh is Portal 2.
Awesome game. Equally awesome DLC.
On a note on FO3: I very much enjoyed the game, particularly in its powerful setting of the Capital Wasteland. I enjoyed NV more, to be sure. It might be that I think it's a better game, but there's also the chance that I played FO3 as a console gamer, which meant my relative skill and ability to appreciate games both sucked.