I chose to stay there for its architectural value. (There is even a Wikipedia article on the place.) It bills itself as one of only three truly Art Nouveau hotels in the world. While I don't know if that is accurate, it truly is a beautiful hotel that they maintain quite lovingly. They should actually give tours of the place as it would have enhanced my stay.
The hotel has a great location right between the beach and the Old Town. It is just a short walk to both as well as to the bus station.
On the down side, I found the included breakfast to be quite skimpy for the price of the room. There just wasn't the variety that you would expect to see in even far less expensive hotels in Estonia. Also, the wifi was shoddy at best.
I stayed in a Junior Suite with a Terrace in the Villa itself. Some of the rooms are actually in the adjacent Gardner's House. If you don't stay in the Main Building, I definitely think that one could do better in Parnu.
Overall, I had a pleasant stay there. But one night was definitely sufficient for me.