This hotel is NO WAY as good as everyone makes out.

I have just returned with my partner and was disappointed, to summarise:

Terrible smell from stagnant water next door the whole trip - made it impossible to enjoy terrace and swimming pool
Beds were really cheap and uncomfortable - had a spring digging into my back (had a comfort room as well)
Staff unable to recommend a good restaurant

On the up side -

Affordable @ 110 Euros per night
Close to a few nice towns
Close to a few nice beachs (in particular the one in the nature reserve - just take food and drink and expect a queue to get in as places are limited (worth it though))
good choice of internal design
clean

Overall - horrific smell ruined the trip ... beds and staff a bit rubbish BUT fairly cheap and clean and not in a bad position

Would not choose again.

Husband, wife and 17 month old booked into this hotel for 3 nights and ended up staying for a week in July 2011.
It is conveniently located about 100 minutes from the Rome airport, very close to several small towns and 15-20 minutes from Monte Argentario, Orbetello and Grosseto (all worth the drive!).
The rooms are modern and comfortable, with huge private decks from which you can enjoy the view of the grounds while sipping some local wine before dinner. We thoroughly enjoyed the gorgeous pool which is huge enough to allow people to relax at one side while an active toddler is splashing around at the other. The staff is very personable and were very helpful and gracious fulfilling all the requirements of having a little one in tow. (playpen, highchair, etc).
Carlotta, the general manager who speaks perfect English, was the perfect hostess with invaluable tips about the local area and a willingness to go "above and beyond" to ensure our stay was perfect.
A lovely breakfast is included in the nightly rate with the usual cereals, fruits, toast and jams but also a generous selection of home made pastries and quiches which were all very much enjoyed.
The absolute HIGHLIGHT of this hotel is the daily dinner at the restaurant. Claudio, the chef, creates magic every night with locally grown, organic ingredients. Vegetarian, fish and seafood dishes are so fresh and delicious, you wont believe you even considered eating dinner off site! Each night the menu changes and includes your choice of appetizer, pasta, main course and dessert. All the food is plated and presented with unique mix of traditional Tuscany and contemporary New York City!
From the location to the pool to the staff, the rooms and the food we had a great week here and overall, the Hotel Fontermosa is highly recommended!

We had a great 5-day stay at the Fontermosa. The energetic staff were very friendly and helpful. The rooms and bathrooms were very comfortable, clean and modern. We loved the quiet atmosphere, and the wonderful and relaxing swimming pool. A great choice if you are going to be in the Grosseto area.

Three sisters stayed for two nights at this lovely hotel. Beautifully appointed, very well serviced with a personal touch. Locationwise: difficult to turn off from the main road.