Using a Visa Debit Card (with PIN) is quite easy. In Dakar, ATMs are not difficult to find. They're ubiquitous in the heart of the city, widely available in other neighborhoods (Sacre Coeur, Yoff, et al). You should find no hidden fees, and the exchange transaction should process at the expected rate.

Even in Casamance there are ATMs in Bignona, near Western Union and Ziguinchor.