The Fallout sequels have evolved into Fallout3, yet the essence of the previous ones is still there in Fallout3. It is not actually necessary to play the previous ones to get that essence feel, Bethesda went to lengths to build on the essences of the previous when making Fallout3, not absolutely identical in lesser details but a worthy and true sequel with great in-play improvements. Early sequels had "room-for-improvement".
I think I'm rather inline with gabriel77dan on this. Personally, I've really enjoyed all of the Fallout games. (Except for Brotherhood of Steel, which not only wasn't terribly faithful to the franchise, but also was a fundamentally flawed game, where both The Bard's Tale (the newest one, with Cary Elwes doing the voice acting,) and Baldur's Gate: Dark Alliance did a much better job with that manner of gameplay on the same engine.) They all have their flaws, but I've liked them all so far (even if to varying degrees.)
I don't think Fallout 3, however, is a direct-line evolution of Fallout 2, however. I see Fallout 3 more as Bethesda saying "hey, you know what would be really interesting to translate into the sort of gameplay we do - Fallout!" I don't think there's anything wrong with that. I don't even think it was a bad decision. But I don't see as how Fallout 3 isn't very much a "Bethesda" sort of game, done in their style, and playing to that company's specific strengths and weaknesses. There were a number of improvements to the series, sure. (Frankly, I absolutely loved the art direction in #3 - and thought it better envisioned "Fallout" than even the original games, oddly enough.) But the gameplay (and not even going into TB/RT, as we're all taking a break from that,) is - like gabriel77dan said - a step to the side, and not a direct improvement. I don't honestly see that Fallout 3 is exactly what Interplay would have made had they been able to continue developing the series this past decade, and been making incremental improvements and keeping up with developing technology and sensibilities.
I thought Fallout 3 was pretty darned cool. I still have a number of reservations concerning my connection to my character, the characterizations of NPCs, the game mechanics and their manifestations within the game, and how well the rule set meshes with the sort of RPG they're trying to make. I feel we really need a Fallout 4 before I see a game where Bethesda has really grown into the franchise, with a clear concept of what they want to do with the game, and the proper comfort level to pull it off. (And that's pretty standard - many game series don't really hit their stride until after a sequel or two.) I think it did some things better than Fallout 1.
But I also think there's some lessons to have been learned from the original titles, that did not fully translate into this new gameplay genre. And that's okay, too.
How does one classify a "true" fan of the early Fallouts but not a fan of Fallout3 or the other Fallout off-shoots. Can they be called a "true" Fallout fan.
For example, I was truly a great Fallout fan of Fallout 1 and 2, when they were out, but now I have Fallout3 I am no longer a fan of those early Fallouts, and would avoid going back to replay them, like the plague, realising the shortcomings in them that has been starkly shown when playing Fallout3, but I still consider myself a "true" fan.
I don't like this concept of a "true fan," either. Personally, I think it's splitting hairs. There are only fans. They come in all flavors. They all like (and dislike) various aspects of these games, and have their own tastes. I don't believe there's any sort of membership criteria for being a fan of the Fallout series. I've looked all through our Forum Rules, and I
certainly didn't see anything in there about having to like or hate specific Fallout games in order to be considered a fan.