I stayed for NYE in 2014 (meaning I checked out in 2015; always confuses me a bit!) and loved it. I'm a big fan of the Disney theming at the other resorts, especially around the holidays, so when I can, I opt for them, but the price difference for a comparable location was staggering. In my opinion, none of the Disney-owned Epcot area resorts are worth $650 a night plus tax. The Dolphin gave me that same location for less than half of that price. And being so close to Epcot right after NYE was amazing!
The Dolphin does decorate for the holidays, but I'll admit not nearly as elaborately as some of the Disney resorts do. But, being so close, I could go grab breakfast (I rarely actually each in the Dolphin) at Trattoria al Forno on the BW and pop in to see those holiday decorations. One night, I booked an ADR at Narcoossee's and got to see the GF decorations.
I enjoyed it so much that I'll be heading down in December for a 5-night stay ending on NYE (heading down to the Keys to celebrate NYE itself with some family down there), but I'd highly recommend NYE at the Dolphin. Great value!