I'd say it's worth playing. It's been a step up in terms of difficulty, with a couple of ridiculous bosses. I've not finished the Ivory Crown yet, but my general impression has been that it's stern but fair. (I've lost far too many souls through my own stupidity, and ledges). I've been on the 360, so not sure if there is a problem with hackers. There are a couple of areas where I have been getting invaded an awful lot but they've never been obviously cheating.
I'm not sure if they are dedicated areas in the dlc, but there are certain areas where you are probably going to get invaded at least every other run.