I haven't had back labour, but my mum, and MIL both did, with entirely different experiences.

Mum had back labour pain becuase my brother was posterior, and said it was quite painful, and in addition to normal labout pain etc.

MIL was reading a book that described back-ache labour, when she figured out that the dull period pain she'd been having was probably BIL coming into the world so got to the hospital quick smart & had him. she said it felt like dull period pain for the whole labour - uncomfortable but not painful, and this was the only pain she experienced. Apparently it was the same with DH.