My boyfriend and I stayed in Casa Mar y Montana for one month this spring. We really felt at home here and would definitely recommend it to anyone looking for a comfortable base in Sayulita. The location is great, even if it is a little ways to the center of town. The walk to the beach is short, and there are shops and restaurants starting right at the bottom of the hill, so you definitely don't feel like you're far away from the action. In fact, one of our favorite places to eat was Purillo's, and I don't think we ever would have gone there if we weren't staying on that side of town!
The room itself was pretty basic, but comfortable and clean and airy - a great place to enjoy the outdoor living of Mexico! One thing to mention is that the ceiling fan in the room was not very powerful. We had a bit of a breeze most nights, so it was OK, but it might get pretty hot in there in the middle of the summer.
The shared kitchen situation worked out just fine, and we used the kitchen a lot. It might depend on who is staying in the other unit, but while we were there, a long-term renter was living next door, so it was really fun to get to know her and get some tips on the house and life in Sayulita.
The hosts aren't on site in Sayulita, but they are responsive and seem to do a good job managing the property from abroad. We had one minor incident where ants decided to march into the kitchen for whatever reason. I emailed the hosts and the property manager and they dealt with the situation immediately. So we felt well taken care of there.
All-in-all, we had a wonderful time. This is a great place to call home while you're enjoying the town and beach of Sayulita!