We (couple in 30's) have stayed here every year for the past three years and we will go back again soon. We come back for the excellent food, great beaches, fantastic weather and friendly people.
The valley itself is very lush & green, largely pine forest and farm, lots of wildlife here from goats to eagles. Generally very quiet, never difficult to get the pool to yourselves or get a good nights sleep.
The apartment is very modern, well maintained and comfortable (very good air-con/white goods/bathrooms etc), short drive from the airport, absolutely stunning and secluded location with excellent views plus a very good near-by restaurant and plenty of things to do.
There is an indoor pool and gym although don't count on the former being available/filled up. The satellite TV is very good although mostly documentary, sport and news channels.
Make sure you hire a car (or use the local buses) and visit some of the local towns and beaches, there is so much to do and see it would be a shame to miss out on them. Highlights for me are in no particular order: Istuzu beach, Dalyan town and Fethiye town/waterfront. Also, Olu Deniz is an hour or so drive away and is a nice place to visit for the day.
There is an awful lot of historical, natural and cultural locations/attractions in the area plus the usual water-sports, boat trips and local markets. No excuses for boredom here!
Negatives? The road into the valley is a bit of a roller coaster (some like me might enjoy this) though the views are pretty stunning. Also, flights are not cheap although the excellent exchange rate makes up for it when you are there.
You will not be disappointed if you choose to stay, a richer experience than the usual resort hotel plus far more relaxing (when required).