The exhibition itself was well-displayed and organised, with lots of accompanying information in both English and French. Remarkably, the actual crania were on display, along with some of the handaxes found at the sites. I loved it, and my family even tolerated it, so couldn't have been a total bore to non-archaeologists. If you're in the area and have 45 minutes to kill, it's worth the admission fee (about 5 Euros).
My advice: skip the permanent collections and go straight to the Atapuerca exhibition. Take the Metro to Trocadero and then follow the signs from there. Don't miss the fantastic view of the Eiffel Tower!