GITS SAC 2nd GIG follows the work of a group known as Section 9: one of those secret, let's-pretend-it-doesn't-exist special ops-type set ups the Japanese government ropes in every time they need help with a spider or something.
Firstly, you don't need to have seen either of the GITS films to fully enjoy the two GITS series. However, if you can, I would suggest watching 1st GIG before this one. If you can't get your hands on it, no worries, 1st GIG is only occasionally referred to, and you can get by just fine without it.
Now, onto the series itself. I'm not sure who exactly GITS is targeted at, as it comprises almost equal parts gunfights, robots and explosions, and metaphors, philosophical discussions and mind-trips. If you enjoy cyberpunk, sci fi, and don't mind the odd long discussion, I would strongly recommend GITS.
2nd GIG follows an X-Files type format: episodes are either standalone (separate, single episode stories) or complex (episodes that follow a story arc). The story arc deals with a group known as The Individual Eleven, who are generally either trying to kill the Japanese prime minister, or each other. It's a little difficult to follow at times, but incredibly well-written and engrossing.
The animation is top-quality, and you will really appreciate those sweeping cityscapes as a character looks out across cyberpunk Japan. The soundtrack is also excellent, and features some beautiful vocals from Origa. As far as voice-acting goes, both the Japanese and English dubs are very good.
So, if you like anime and appreciate the classics: buy this DVD. If you're a sci fi fan who's willing to give anime a try: buy this DVD. Lastly, if you've already seen the films and 1st GIG: where have you been? Buy this DVD!