I have been enjoying Fallout 3 for about 70 hours now and I am having as much fun now as I was 50 hours ago, which is to say A LOT.
I cannot speak to the bugs that others seem to be experiencing. In my 70 hours I have had my game freeze only a handful of times, and none of them were anything that was game breaking. I have several friends who are also playing and none of them have expressed any serious glitches either.
I have played both the previous Fallout games and vastly enjoyed them both, so to make a blanket statement that "anyone who was a fan of the first 2 Fallouts will hate this game" is just erroneous. I would agree with someone who said this game is more of a Next Gen title in the Fallout Universe rather than a true sequel to the Interplay games.
However, that does not in any way detract from the epic amount of fun to be found in Fallout 3.
The adventuring around the Wastes is the highlight of the game to be sure. The amazing amount of freedom to roam the Wastes is just phenomenal, so much so that once you do decide to progress the Main Story it can seem very limiting and cramped at times, especially when crawling around through the DC Ruins and all the subway tunnels required to get to the Main Mall.
However, compared to the vast scope and size of the experience, this is a minor complaint and really a very short and small portion of the game. It should only take maybe 5 hours to explore it all and finish up some questing, then you can Fast Travel back whenever you want and get back to tearing it up in the Wastes.
Combat is quality, but it is NOT a Shooter. People expecting Shooter mechanics will be disappointed, as even point blank shooting still utilizes an in game Dice Roll to determine whether you hit and how much damage you do etc.
It is likely that people complaining about Ammo or clunky targeting are unaware of this game mechanic, but it is what makes the game an RPG, not a Shooter. There is ample amounts of Ammo (I have never run out in 70 hours of gameplay and I have killed over 1000 living -isms according to my in game stat sheet), but it is not unlimited like a Shooter.
The VATS mode is a good way to illustrate the difference. You queue up VATS and it will give you a stop-time targeting system. Within this system, you will be given percentages to hit on various Body parts: 87% to hit the head, 95% to hit the Body etc. However, these percentages are also indicative of how much of a chance you would have to hit those body parts OUT of VATS as well. So if you have a 50% chance to hit a leg in VATS, you have that SAME chance to hit in real-time it just does not illustrate it anywhere. SO even if your targeting arrow is Red, you still only have the same percentage to hit as you would in VATS. Which is to say you will miss and waste a ton of ammo if you just think "my target is red I can unload a clip".
Dice Rolls are a staple of RPGS, Oblivion had them, it was just not as noticeable because you were using Melee weapons, but every time your guy reared back to swing it rolled the dice to see if you would hit, how much of a damaging blow it would be etc.
The game has a wide array of voice acting, some is great, some not so much. The story has its ups and downs, but is good enough to not take away from the overall (it certainly helps that the great Liam Neeson is the Vault Dweller's Father, he is an example of the Great voice acting). Various mini-quests are actually more entertaining and interesting than the main story, so there is definitely enough Good story in the game to keep it interesting.
However, the game's strength is in solo-adventuring IMO. While you can get sidekicks and helpers, the real fun is being a Hero of the Wastes and taking down Super-Mutants, Raiders, Slavers and Death Claws all by your lonesome as you scour the Wastes in search of unique weapons, caps, ammo and mini-quests littered throughout.
Overall, I would rate the game a 9/10 and consider it the only game on Xbox 360 that gives Mass Effect a challenge for best title.