There's a couple of things worth keeping in mind with Fallout-3.
For one it is not a shooter really, it has guns and an FPS view, but people who went into the game thinking they would get a serious shooter where very disappointed.
It's and FPS-RPG, and you definately need to approach it as an RPG where the combat just happens to feature guns instead of the usual swords and magic, you should definately not be playing it for the shooting mechanics alone, because they are not the game's strong point.
Nor is the game's main story line, it's kinda.. well it's not great, it starts out just fine, but about half way through it kinda seems to lose momentum, and it only gets less engaging from there, and ends on a downer.
What you really should be playing it for is the exploration and all the side quests, there really is a lot of content there, it's not all great, but some of it it is very good, and just the scale of the world and the exploration you can do is awesome if you like that sort of thing in games.
It's extremely buggy and glitchy, save constantly, you never know when the game will crash, or when a quest gets screwed up over some minor glitch. It's also very fickle about what hardware/software combinations it'll run well on, i had tons of crashes and glitches, others have had much smoother experiances with the game, and nobody has been able to pinpoint what makes the difference or fully fix it.
Mods make the game, there's a huge catalog of mods for it, and you definately need to look into them, there's some really good stuff out there.
Also, Fo3 is not a great sequal to 1 and 2, not terribly faithful to them if you will, and that clearly cheesed off a lot of Fo fans. I took it as a standalone experiance, and that definately made it better.
So yeah, it is a very flawed game, but if you play it for the right reasons, it's also a very good game, play it for the wrong reasons though.. yeah, there are people who really despise this game, most of them beeing the fanbase of the origionals it seems.
But if you can take it as a standalone game, and you can live with the following things, then it could be worth a purchase:
Do you like sandbox exploration? Because that it does, and does really well, and it's the main thing i personally like the game for.
Can you accept that the main storyline will be something you just do a mission or two of every now and again, between exploration and doing side quests? Because that seems to be the best way to play it, the main quest isen't good enough on it's own really.
Can you accept that it's gonna need some mods to be at it's best? Because that is definately the case, after playing it with a few mods that appealed to my tastes, i could never go back to playing it vanilla, it's definately a modders game, much like Oblivion was.
Can you accept that it's not a straight up shooter, but should be played more for the RPG elements? It's not a great shooter..
If the above sounds ok by you, then yeah, it may be worth the purchase (infact the only reason i won't say "then you should buy it" is the buggy nature of the game).