Fallout 3 does a lot of thing's right, but it still trips up in several key areas.
I may sound negative about Fallout 3, but trust me it's a damn fun game. The game begins with your character's childhood, going through the early teens, and beginning the game-play proper with your 19th birthday (this is a trend that I am going to discuss in my next blog post). Your character is anything you want him/ her to be, from a guns-a-blazin badass to a stealthy assassin. The core gameplay of Fallout 3 is either a basic FPS, or a more turn-based shooter. The turned base gameplay is used through the V.A.T.S. system. V.A.T.S. isn't particularly deep, but it is very accessible and does it's job very well.
The world of Fallout 3 is very impressive, in that it is massive and highly explorable. Also, there's a LOT to do in post-apocalyptic Washington, D.C. I honestly had the most fun just wandering around finding locations and fighting random Raiders. Also, the soundtrack is very fun, as are the other 50s homages, although the retro theme was pulled off better by Bioshock.
All of the things I mentioned above would make calling Fallout 3 not great seem like blasphemy, right? Well...wrong. All of the parts of Fallout are great, but they just don't mesh together to feel like a great experience. V.A.T.S. works very well, but it felt as if it were a tacked on addition in order to keep the traditional Fallout turn-based gameplay. Also, the traditional FPS combat is playable, but very finicky. I'm not sure if everyone else will feel this way, but it was my impression that the world of Fallout 3 wasn't alive. Yeah, it is the apocalypse, but couldn't there be more interesting locales and more unique individuals inhabiting the wasteland, and not just the towns? Finally, the gameplay of Fallout 3 allows yu to be stealthy, but it also makes this to be a very unexciting, frustrating experience. I suggest you avoid stealth at all costs.
All-in-all, Fallout 3 is great fun to be had. However, the issues I found in it detracted from the overall fun factor of the game, and thus I can't dub this a game to sit along side Oblivion and Bioshock (games I feel Bethesda was aiming to have this game compared to). SO checkout Fallout 3, but don't be expecting a masterpiece on the scale the hype may have lead you to wish for.