I've have stayed there as a renter several times during ski season over the past few years, and purchased a 3 bedroom this past March.
The hotel is located at the end of the Killington access road, right at the base of the Snowshed slope. In foliage season you will be able to take a 2 minute car ride over to the Killington base area and ride the Gondola up to the top.
Most three bedroom units are located on the top (3rd floor) of the hotel, although there are some scattered in other areas. You either will have a great view of the mountains, or a parking lot/golf view.
The highlight is the pool/health club and brand new spa. Great view of the mountains from the pool. In winter, you start inside and swim outside. But it is wonderful (and heated) all year round. Nice sauna and steam room. Very good workout facility. And brand new spa with high end furnishings and treatements - moderately priced. Book your appointments ahead.
The Grand is in the process of complete renovation of the guest rooms. New furniture, interior painting and soft goods (comforters and drapes). The top floor will be addressed first, but even if your unit is not done when you arrive, they are still in very good shape.
Killington tends to be quiet during non-ski season, but there is still lots to do. Spend a day in Woodstock (about 35 minutes away) or Queeche (another 10-15 minutes past Woodstock). For fine dining, try Hemmingway's - it has a national reputation. Lots of other very good choices for dining too.