*WARNING - early game spoilers in this post*
I'll be damned, maybe only 3 hours or so isn't enough to start with the hyperbole, but I do believe Bethesda have actually nailed it.  I'm probably the least skeptical poster on all of GAF (I don't passionately hate Oblivion, and for that matter, I don't passionately hate everything that isn't Fallout 1 and 2), but I'm still finding reason to be impressed here.
On the bad side I was surprised to find the game really is superficially just Oblivion with guns.  I argued a couple of years back that it wouldn't be, but sometimes it really does just feel like an expansive Oblivion total conversion.  However, I have been pleasantly surprised to find that this doesn't detract from the experience at all.
One thing's for sure, the game really tries to suck you in.  When I started the game, I was in the "I'm playing the tutorial in a game" frame of mind.  When I switched it off some time later after escaping the vault, exploring Megaton and it's surrounding areas for a bit, then walking across the wasteland for a bit on my way to a quest, I was the child of an underground vault set loose on a nuclear wasteland of an American city.  I was there, man.  That doesn't happen to me too often.
Didn't take long for me to feel like my actions made a difference to how the game played out, either.  The character I'd roleplayed for myself was a bit of a geek, a bit of a do-gooder... at the start of the game I'm almost at the Overseer's office when I walk past some kind of cell room where I see him with a guard questioning his daughter about the disappearance of my father... the situation looks like it's about to explode, as she pulls out the gun I let her keep earlier, telling them to stay back... 
For whatever reason my gut instinct has me sneak into the room behind them, landing a crit on the skull of the Overseer when he least expected it... my bat strikes him with a sickening crack he flies forward in slow motion, his now lifeless body crumpling over a nearby computer terminal... I frantically beat down the guard... finally we're safe, but she is just freaking out on me for killing her father, even though I had thought I was saving her life.  
At that point, I paused the game, and quietly whispered "wow!" to myself in amazement.  There are so many ways that scene could have played out, based on what I'd done earlier and how I handled it.  This is roleplaying.  I can only hope the rest of the game holds up to these kinds of standards.
I was completely blown away when I finally got out of the vault and looked over the wreckage of the world in front of me, too.  I didn't expect it to strike such a chord with my emotions, considering I'd previously seen screenshots and videos of it.  Let's just say, there's a world of difference between looking at some screens of this game, and actually experiencing the world first-hand.  The initial time I spent slowly wandering around the little town just outside the vault, with the voice of the President and the music on Enclave radio playing in the background, it was just so chilling and atmospheric.
However...
The game does suffer in some of the same ways Oblivion does, and it's mostly down to the way the AI behaves and reacts.  Probably much less so than Oblivion from what I've seen, but the world is so perfectly realised here that it's possibly more jarring.  For example, within the first 5 minutes of leaving the vault, a scavenger just happens to walk my way.  This is my character's first contact with any other human being right outside the vault.  I engaged in conversation with the scavenger, thinking it to be quite a profound moment.  But no.  The scavenger said hello and my character calmly asked if they could repair weapons.  Nothing at all about the fact that I had only just seen the sky for the first time 5 minutes earlier.  Broken immersion, right there.  
Once I got to Megaton, Oblivion syndrome set in, by which I mean NPC reaction to me was all over the map.  You've played Oblivion, you know what I mean.  "I hate you, if I ever see you again I'm going to kill you, nice talking to you come see my if you have any more questions".  When the NPC interaction works, it's superb, but when this stuff happens it pulls you right out.  You'd think they'd have some flags in place to stop this kind of thing happening, considering how detailed other areas of the game are.  It's probably even more jarring than it should be since I've been playing Fable 2 recently, the NPC social interaction in that game is amazing.
Wow, this post is getting long.  Says a lot about the game, I guess.  Quickly - I had some trouble adjusting to the mix of real-time and VATS, I really wasn't liking it (other than beating those ant things to death with my bat in slo-mo, very satisfying)... at the end of my play time last night though I was starting to get the hang of it, I'm looking forward to spending more time getting to grips with it now.
Anyway, so far so good.  Nice one, Bethesda!