It is not an entertainment-type 'party' so much as it is a private viewing and dessert-dining experience.
It is held in the lower level of the Tommorrowland Terrace CS restaurant. The Terrace is closed to everyone except party guests. The event starts 1 hour before the night's fireworks. Each group has a reserved table (a real reservation, not an ADR). You cannot really see much of the Main Street Parade but you have a beautiful, unobstructed view of Magic, Memories & You and then of the fireworks. Tink flies in right over your head. You can stay seated at your table or move up to the railing if you prefer, but there is enough room for everybody because attendance at the party is quite limited.
From the time you enter the party until shortly after the fireworks end you can help yourself to a dessert buffet (fancy tarts, little miniature desserts, chocolate-covered strawberries, etc - not traditional 'kid' things like Mickey bars). Beverages include hot coffee, iced tea, lemonade, and milk. Soda pop is NOT on the buffet.
Whether it is 'worth' it completely depends on what you and your group would enjoy. It is much more expensive than, say, buying a big ice cream cone or a pastry from the bakery and watching the fireworks from Main Street, but we really enjoy the chance to sample so many, many desserts. And to drink GALLONS of ice-cold milk. The parade is not a deal-breaker for us. Getting to sit and talk quietly without staking a spot out hours ahead of time is grand.
There have been lots of threads with pics; search for them or search dining reviews. Do NOT attempt to eat dinner on a night when you are doing the dessert party. Have a big lunch instead, at least 5 hours beforehand (so no later than 3 pm if the fireworks are at 9). It's bonus if you schedule the Party for a night with evening EMH; that way you are all 'fueled up' and the sugar high from the Party will easily take you through to 2 am or so.