Unlike most sequels, truly better than the original - although with continuity and maturity problems
Many of the best things from the original were kept and improved upon: Although there was a main plot, you really did feel that you were travelling and exploring a complete world. You had real, meaningful choices in character design and development, both mechanically and in roleplaying - and your actions toward NPCs had realistic lasting effects - without an artificial "alignment" system. Virtually any character development path was entirely playable, even to the extent that a reasonably clever player could complete the whole game with a pure diplomat player, barring bad luck on random encounters. The worst mistake of the original - the hard gametime limit - was avoided cleanly.
The larger map and longer gameplay did expose the biggest weakness of the Fallout team - their sophomoric (and arguably misogynistic) mentality. While much was made of the fact that your character could actually get married in the game, in fact the only marriage available was a shotgun marriage to a useless wimp, and all the roleplaying choices related to the spouse after that emphasized a really grotesque parody of the worst and most misogynistic teenage male ideas about it. The clever melding of memes and imagery from the Cold War 50s that characterized the first game was exposed as a happy chance, as the sequel broke continuity in various ways (especially in computer technology) and threw in some trendy antiauthoritarian government-conspiracy themes. The original story seemed to be set in the future as imagined during the Cold War, and worked very well. The sequel's backstory is a pastiche of 1998-era X-files influences, random gaming memes, sophomoric humor, and shreds of the original, that really doesn't hold together unless you forget the original entirely.
The Bottom Line
Purely from a gameplay standpoint, it was an excellent game, better than the original and a lot of fun. From the standpoint of storytelling and worldbuilding, it was clumsy, sophomoric, and terribly disappointing. If you have the chance, play both Fallout and Fallout2 in proper order. They are landmarks in real roleplaying on a computer, and very fun to play. In particular, if the shoddy worldbuilding and sophomoric culture and character portrayals of Fallout2 overshadow the fine gameplay for you, you might find the mix in the original game more to your taste.