I have been twice to Dakar for business, and enjoyed it immensly. First time, I stayed at:
Very interesting (positive) experience and the restauant down by the oceanfront very nice.
Second time I stayed at the Pullman Hotel.
Couple of points:
Ask the hotel to arrange for a car to pick you up from the airport
Use taxis from the hotel
I did not feel at all un-safe at any point