We just got back from Djerba and can really recommend both this lovely, small boutique-style hotel and the island. The hotel is situated right on the sea front, so you'll have beautiful views of the sea from you balcony and will be able to go for a swim at any time of the day or night. The hotel has very friendly staff, who are always willing to help and give you the best service. This hotel has a very homey feel, due to it's small size (it only has 17 rooms) and this is great when you want an alternative to the big, more impersonal hotel chains that dot the coast-line. The hotel owner speaks excellent English, French and German so no problem with communication and telling him exactly what you're after. Definitely ask the owner to pick you up from the airport, it's the easiest way to reach the hotel and a lot more interesting that a taxi cab. You'll find the food in the hotel very authentic and tasty and the fresh orange juice is delicious. There's also a supermarket around the corner if you need anything extra or if you've left behind the suncream.
As for the island, it's a great place to go when you just need to get a bit of piece and quiet and some sun and sea.