Plattekill is THE place to go in NY during a good snow year. If there's a solid base down, Plattekill will be going off. They offer an "if you can see it you can ski it" policy, and the resort and volunteers maintain a ton of interesting, technical gladed trails which you have to do some research to find. This is definitely a locals mountain. Meet somebody on the lift who skis here regularly and get them to show you the best spots. On a powder day, Plattekill can't be beat for gladed terrain and a great, old-school ski bum locals atmosphere. The bar in the lodge is a great place to keep the fun going as well. It's got a great old school vibe going for it, and is definitely a unique spot.
When the conditions are right, Plattekill can rival any mountain on the East Coast for terrain in terms of steepness and technicality. It's smaller, but rides much bigger due to its terrain policy. That being said, being in the Catskills and much further south than mountains in Vermont, NH or Maine, it doesn't get the same opportunities for snowfall, so timing a trip can be difficult. It's also on the far west side of the Catskill Mountains, meaning it gets the most snowfall annually of all the Catskills resorts (about 200 inches/year on a good year) but its also far from... everything.
All in all, Plattekill is a great, unique gem of a mountain that provides an incredible experience during high snowfall years. Terrain and atmosphere combine to make this a must visit when Mother Nature provides the goods.