Wow. Just... wow. There's lots of stuff to like in Fallout 3, but I think the thing that strikes me the most is how engrossing an experience it is. This is the kind of game you can lose yourself in, only to look up at the clock three hours later and kick yourself for missing bedtime... that's pretty rare outside of the strategy genre. Fallout 3's post-apocalyptic DC landscape is beautiful and unique. It's a joy to quest or just explore familiar and unfamiliar landmarks, dodging mutant and raider gunfire as you go. There are tons of different ways to solve every quest, and most of the quests have a surprising amount of depth. NPC death is permanent and may come as quite a shock when your choices have unexpected outcomes. The desperate setting comes across nicely- you will rarely have enough ammunition to truly feel comfortable, but dedicated scavenging will keep you from resorting to fisticuffs. Your weapons and armor will always be in some state of disrepair- everything in Fallout 3 comes second hand, and maintaining your equipment is a critical part of surviving the Capitol Wasteland's myriad horrors. You are not an invincible supersoldier, immune to the hardships suffered by DC's other residents. You are a Wasteland warrior, battling rivals for resources, as charitable or mercenary as you please, struggling to survive in a hostile world that may (at least at first) frequently succeed in killing you.
It's astonishing just how much there is to see and do. I've spent somewhere in excess of 7 hours on the game now and haven't progressed the main quest at all, aside from speaking to some people in the local shantytown. The leveling system is much improved over Oblivion: virtually all the available skills are immensely useful, and you can select a new feat-like "perk" whenever you gain a level. Perks range from the practical ("strong back" increase the amount of salvage you can carry) to the vicious (the "bloody mess" perk results in exactly what it sounds like) to the bizarre (the "mysterious stranger" perk randomly summons a silent gunslinger who fights at your side, then disappears). It's quite possible to develop a character perfectly tailored to your playing style.
I can't comment on whether Fallout 3 is as good as its predecessors, but I came to Fallout 3 with high expectations... and they have been exceeded. Like RPGs? Like first person shooters? There's a little bit of something for everyone in here... except for kids. Whatever you do, don't let children anywhere near this game. It may be awesome but it definitely earns its 18+ rating.