We chose Mango Plantation for our recent stay in St Lucia. Its a lovely, authentic old plantation house, and was perfect for our needs. The house offered all we needed for our stay, and the garden and surroundings were perfect for relaxing and enjoying the sunshine (and humming birds). As others have mentioned, access to the house is not straight forward, the roads are quite challenging, and a 4x4 is strongly recommended, although we just about managed with the normal car that the hire company delivered. The trip to Soufrieres is well worth making, and Pigeon Island was also good. Castries itself is a little disappointing, but the market offers fresh fruit and veg, and there is a supermarket next to the market for other provisions.
Cathy is a good host, and made sure that our arrival went smoothly, and was on hand for advice and help on what to see and do. She also kindly gave us a guided tour of the local surroundings, which was a massive help for us finding our way around.