The second series of Durarara is a pure and true expansion the original series, mostly due to the fact that writer Ryogo Narita never intended the series to end where series 1 ended, but also how well this series replicates and expands on the original in story and vibe.
As it starts where series one ends, i WILL spoil "Durarara!" here, but not "x2".
The original series was about Mikado Ryuugamine, a young man whom started The Dollars, an internet gang that got out of control as passwords were leaked to forums and Izaya Orihara gave the information indiscriminately to anyone, and how his internet gang raised paranoia within traditional "color" gangs.
His best friend, seeking friendship and purpose after moving to Tokyo, started the Yellow Scarves, a turf-based color gang in Ikebukuro. When Mikado moved to Ikebukuro himself, both resolved to keep their allegiances a secret and unintentionally start a war between both, with Anri Sonohara caught in the middle as the Slasher, formerly another girl in possession of her power but whom was unable to control is as Anri does.
All allegiances revealed to each other, they kinda split off to try and solve their problems and thats where we start off.
Some new faces get added to the mix and they largely drive the plot into gear: freshman student Aoba is trying to get close to Mikado, knowing the latter is a Dollar, Izaya's younger twin sisters Mairu and Kururi also get into Raira Academy as freshmen and end up with a month of Celty's pay after the latter drops it during a chase, and two Russian assassins, Slon and Verona, end up in Ikebukuro with a stolen anti-materiel rifle and an unknown purpose.
All this occurs while other plot threads from the original are continued, such as Nebula's buyout of Yagiri Pharma, Shingen Kishitani's dealings with both, a Yakuza deal between families and Izaya's plot for Celty's disembodied head.
Animation is a little better then the original and the voice cast is the same for both English and Japanese, and the dub is solid. Soundtrack is also really interesting and eccentric.
