The local currency dalasi (D) fluctuates strongly. There aren't any official changing points at the border, just very persistent black-market changers. You'll be fine using CFA, though, until you get to the coast, where changing money is easier. Many hotels can recommend an informal changer, though the rates may be similar to those the banks propose. Many hotels will accept UK pounds sterling.
There are no ATMs upcountry, and you're best off changing all you need at the coast.