I only used the BF for the traders. That was partly because I wasn't really interested in straight hack and slash, and partly because of complaints in the forum (I wanted to prove it was unnecessary to level up in the BF, so the dungeons were entirely optional).
Act 1 was designed to be fairly linear, but 2 and 3 open up more, and are mostly above ground. Act 4 is indoors, but doesn't feel like a dungeon crawl (there are dungeon type areas, but also adventure game elements in the NPC interactions and quests).
