Used to be ok-ish, now seems to be largely publishing unrevised PhD theses without much if any quality control. If it's a question of getting a book from a thesis,and you want an academic career, it's worth putting the work in to place it somewhere better. Peter Lang seems to be regarded in the US as a vanity press, in Europe it's not seen that badly, but still in my social science field at least, it would be near the bottom of the list of presses to try. But still better than VDM/ LAP and its multiple offspring based in Saarbrucken, so everything is relative.