Loved it, although it is definitely not for everyone.
No matter what the Dining Reservation phone line or the website says your ressie time is, it actually opens 1 hour before the fireworks start and the CMs ask you to be there 5-10 minutes before that. They pre-check you in, then can immediately escort everyone to their tables at the official open.
You have a table with your name already on it (no need to camp out 3 hours early for a "good seat"). There are several buffet tables set up with a wide variety of small fancy desserts. There is a limited beverage selection (how often can you find bottomless refills of ice-cold milk?) but last year the drinks did not include soda pop; that's a problem for some people.
You can not really watch the Main Street parade from the Dessert Party location. If seeing the parade is a deal-breaker and this is your only evening at MK, don't do the Dessert Party.
When the fireworks start some people naturally move up to the railing (some stand, some drag chairs forward). Others stay at their tables. Tink flies right in over your head to start Wishes; be sure to have your camera ready (oh, and a Photopass photog was hanging out there before the fireworks; some people got a lot of shots with him beforehand). You are not dead center to the Castle - the absolute best view of the fireworks is from the middle of Main Street - but it's still very good and completely unobstructed by other people. The folks moving up to the rail are not a problem; lots of room for everybody.
Shortly before fireworks start I noticed that they began to consolidate the buffet. Originally there are multiple repeats of all items, which is good because everyone is starting all at once. But they did keep a single offering of each dessert stocked clear through the end of the fireworks. If you move up to the rail, just keep an eye on your table or tell the server that you are still working on your food/drinks and they won't clear it. In fact, when the fireworks are done, the buffet remains open for at least another 10 minutes or so. Let the crush of the crowd pass by while you refill that coffee cup and grab a last chocolate-covered strawberry!
For us, it was great to sample so many different desserts and to sit in a more peaceful setting. For someone else (especially with younger kids), they might just rather have one great big ice cream cone and have the higher-energy experience of being in the Main Street crowd.
One last warning - when you make the reservation they charge your credit card, just like HDDR. Hope all this info helps; like I said, we really enjoyed it!