What are the additions in the Director's Cut?

There's an entire subplot that wasn't in the theatrical cut. Less romance between Matt and Elektra (their love scene was taken out, and the end of Matt and Elektra's rainy scene on the rooftop was completely changed -- instead of ignoring the crime he's hearing to go sleep with Elektra, he leaves Elektra to deal with the crime). There are a number of extended scenes, mainly the fight scenes.

I say it's worth it. It's not like it's still being sold at full retail price anyways. The BluRay is like $9 brand new on Amazon.

Definitely worth the effort. Besides a better, more involved, grittier movie there is a classic documentary in the extras; Giving the Devil His Due. Avi Arad and the other producer, Forster turn themselves in knots trying to justify their butchering of Johnson's script/movie for the theatrical release. It's a real eye-opener.
Goddam suits, man.