I went with two girlfriends and one male friend. We LOVED Mauritania. The trip there via public transport via Senegal was long AND hot, but well worth it. We had some problems at the border, as my friend tried to bring in local currency we bought in Senegal, which is illegal. So we had to pay some bribes. Be careful of locals trying to be "friends" with you and offering help while crossing the river, as they're likely to try to scam you. You take a car into the capital city and will be dropped off at a gare routiere. Finding accomodations is easy if married or alone, but a group of friends (male and female, not married), can be a problem, as some hotels won't let you share a room together. Throughout the weekend, strangers were often asking us if we needed help and offering it.
Unlike Dakar, Senegal, where people always want something from you or are harassing you, Nouakchott residents are helpful, friendly, courteous and generally just want to give you a good impression of their country to take home with you. The markets are wonderful, quiet and there's a lot to buy. It's fun to buy a local "veil" that you can drape around yourself, showing respect to the locals. The beach is quiet and almost empty, fun to take long walks alone to relax.
The best food is found by accepting offers from locals to come home with them for a meal. You'll likely eat rice and meat with your hands (before which they'll wash your hands with a teapot-like thing for you) while sitting or laying on the floor. Men are able to be more open with foreigners than the local women. But most men are not acting out of sexual interest, just curiosity and friendliness.