Not unpleasant, Rio Branco feels like a trading town, not unlike one of the remote South African cities in Mpumalanga or one of the Capes. The climate is comparable, the people look similar, the town's architecture, functional, low level constructions, could be either here or there.
Rio Branco is, as far as Brazil goes, in the back end of nowhere. Only one of the continents capitals, Montevideo
, is further away from Rio Branco than Rio de Janeiro
is, and then only by a few kilometers.