I took my wife to Cumberland for her birthday. I had been to the island but not the Greyfield. Yes, it is expensive. However, yes, it is more than worth it. Middy and Mary Jo who run the Inn are absolutely wonderful and have a staff that makes you feel at home, just like you are visiting family. Staying at the Inn makes you feel like the Carnegies must have felt.
I encourage you to stay in the Inn and not the cottages - - gives you more of the feel and experience. Getting to know the others is half the fun! Don't worry about shared bathrooms. Yes, I was worried. But it actually was fine. Everyone was courteous and thoughtful. Just plan a little so you don't take too long. We actually used the bath house behind the house, putting on our robes and crossing the driveway in 34 degree weather, and it was absolutely wonderful.
I recommend the Master Bedroom though we never used the separate sitting room. If we wanted to sit and read, we went downstairs by the fire so we could meet others.
We ended up staying another night because it was such a special place and had to switch to the North Marsh Room. Though no where near as large as the master, it was perfect as well with tons of windows and light. I actually enjoyed waking up to taps being played at the St. Mary's Naval Base. It is amazing how well sound carries across the water.
Four hints. Stay for a Saturday during oyster season so you can enjoy and oyster roast but wait to get dressed for dinner until after the oysters. Second, take the tour of the north island and make sure you plan it on a morning when Plum Orchard is open so you can tour the house.
Three, save up a little more and walk over to Gogo Ferguson's studio for a special piece of jewelry for your wife. It is a reminder that she will never take off and so perfect to remember the island. Four, use the bikes. Go to Dungeness, ride on the beach and explore the lower island. The bikes make it so easy and fun.