I had a lovely overnight stay at Charlotte's place. It's a little bit out of the main square, but in a quiet spot, and things are generally pretty close-by in Martinborough.
The space was clean and open plan. Had all the basic things you need for a short stay, with the added touch of a nice continental breakfast kindly provided by Charlotte. There aren't any cooking facilities or a kitchen sink, so not ideal if you are wanting to cook at home. Would recommend for an overnight stay if you're exploring the wineries.