I've attended the last two years. This is considered a merchandise event much like Disneyana was. You pay Disney for the opportunity to buy HM merchandise. There is a gift for everyone (last year we received a sculpture of a grave and headstone - quite cute).
There is food served but only certain rides are open during the event. Last year it was the HM, Big Thunder and Pirates (only because Splash was closed). The best part was when the Headless Horseman rides through. Unlike the beginning of the
MNSSHP where he doesn't stop, here he does.
I've attended with friends and we always have a great time together.
Search Happy Haunts Ball on ebay and you will probably see some of the merchandise.
This is not like the Haunted Holidays event at DL where they makeover the HM into nightmare before christmas.
If you collect HM merchandise, I'd say go for it. Otherwise, I suggest MNSSHP.
Cyn