We stayed at Erratic Rock II (sister hostel to Erratic Rock) in Puerto Natales. ER II is for couples whereas ER is backpacker.
The family who ran ER II were friendly - even got to practice my spanish. There is luggage store to store bags whilst trekking in Paine and the rooms are chalet style - ensuite double rooms. The room was clean and comfortable inc breakfast.
You can contact ERII through www.erraticrock.com.
As we had limited time we booked our trip to Paine in advance - I would recommend Path@gone agency - www.pathagone.com as they were brilliant in sorting out our accommodation and activities in the national park. we did the w trek - 2nd day horseriding. The French Valley is well worth the detour up - it's amazing in good weather!