I put 200 odd hours into Fallout 3, it was an amazing experience, and I still go back into that world occasionally just to putter around.  I still haven't beaten the last couple of pieces of DLC, I should definitely do that, but now there's Fallout New Vegas.
 
I spent about 3 hours playing yesterday, and woke up 2 hours before I needed to leave for work this morning and jumped back in again.  The game has crashed on me twice, not a horrible crash:hours played ratio when it comes to games based on Bethesda's engine, but still annoying when it happens.  If Fallout 3 taught me anything, it was that F5 is your friend, your very, very good friend who doesn't let you walk for 40 minutes, kill a dozen powder keggers and a handful of giant ants, only to send you to the desktop with a vague error message that loses all the XP gathering.
 
I've gotten pretty spoiled in Fallout 3 with the hi-res texture mods out there, so New Vegas seems a little drab in comparison.  Still not a bad looking game, and I knew what to expect, but I definitely look forward to one of the great contributors are Fallout Nexus releasing a giant texture pack that will make it look even better.  I have a fairly beefy PC (I7-920, GTX470, 9GB of RAM) so it runs great, I haven't seen the framerate issues that others have experienced when NPCs are on the screen, but I have a few friends with that problem, hopefully Bethesda/Obsidian will get that ironed out.  For me running at 1920x1080, Ultra High, the game is very smooth and consistent, other than the crashing.
 
My favorite thing about the game so far is that it's had a lot of genuinely funny moments even in the 5 or so hours I've been playing.  Fallout 3 was a great game, but it wasn't funny, even the one moment in the game with the guy and the naughty nighty was just more awkward in the context of the situation than anything else (and it didn't help that it was broken, since I left the nighty in the case it  was in and he was still demanding it even tho I didn't have it).  New Vegas has made me smile several times already, almost every mission seems to have either an outright play on a given situation, or a tongue-in-cheek moment that reminds me of Fallout 1 or 2.  There's a lot of Obsidian in this game, a lot of what was missing from Bethesda's Fallout game.
 
The voice acting is better than it was in Fallout 3, not always exceptional, I was going along, really impressed, then one of the women in the bar at the Mojave Outpost brought my expectations down a bit.  The women in general in the game feel way too over the top for their own good, it feels like everyone is Calamity Jane from Deadwood.  I don't mind foul language, but I also think enough is enough, and too much is too much.  So far it's too much, especially considering it only seems to really be the women who bring it.   Otherwise, the voice acting is definitely a step up from Fallout 3.
 
There seem to be a lot more enemies in the game this time around, especially out in the wild traveling between towns and outposts.  it's also nice to see groups of enemies clumped together, it definitely provides more of a challenge and adds more strategy to the game that Fallout 3's 1 or 2 at a time approach lacked.  Last night I found myself trying to whittle down a group of 5 Powder Keggers and getting owned over and over by them.  It made me spend a lot more time planning out an approach and deciding which weapons I would be using against them.  There have been at least 3-4 situations where I was fighting off 4+ bad guys at one time, which is great.
 
The quests seem a lot beefier and satisfying.  5 hours is still pretty early in the game, and I have yet to hit level 3 yet, but the missions I've taken on have seemed very much in line with the atmosphere of the world I'm moving around in.  There's definitely a more defined sense of people wanting you to prove yourself before they give you anything, which is something else Fallout 3 was missing.
 
So far I'm totally absorbed by this game.  It's definitely on track as being a much better experience than Fallout 3 was, and I loved me some Fallout 3.