I stayed there three days in December 99, and although all inclusives are not our scene, we liked the Riu Mambo. The situation is very beautiful in Bahia Maimon and with rough hills around it. It will take you about 10-15 minutes to drive into Puerto Plata.
Food was generally very good and with lots to chose from (buffet style). And there were not any crowds or queues while we were there.
Rooms are fairly large with a balcony. Pool area is nice but entertainment starts every afternoon and I am not really into that kind of thing, so we prefered the beach.
The beach has brown soft sand. There are quite a few rocks in the sea but they are easily avoided. It is a good beach for children and there are plenty of beach chairs.
Beach activities are a 10-15 minute walk from the hotel in a neighbouring bay. This is actually a good idea as it gives peace and quiet to the swimming beach. You don't have to watch out constantly for motor boats, wind surfers etc. Most activities are included.
Plenty of activities for children and adults alike. Disco on the premises. We never tried it, but others found it good fun. For nightlife I would probably venture into Puerto Plata.
Service was good, and the whole place spotless. We found it a good place to stay for the last days of our holiday in order to digest all our impressions from travelling around in the Dominican Republic. However, the danger of all inclusives is that you don't get to see much of the country, and that would be a pity.
If you have the time and the money you ought to take one of the express buses from Puerto Plata to the Samana Peninsula and stay there a day or two. It is very beautiful. Also from Riu Mambo you can take day trips into the mountains, another "must". Don't bother with paying for tours of Puerto Plata. You can do it yourself cheaper and easier and at your own pace. On the way to Puerto Plata you pass two small neighbourhoods, Cofresi and Costambar. Cofresi has a nice beach and some nice restaurants. Costambar is mainly a residential area with nice restaurants as well and very quiet. There is a nine hole golf course in Costambar.
From Riu Mambo you can walk to the little hamlet in Bahia Maimon. It is an easy morning or evening stroll with great views along the way.
I hope you will have a nice time. You can find more reviews of the place on Debbies Dominican Travel Pages (follow link below).