I think it depends. I've had a non-technical PM and he was great. He knew he knew nothing about development and as such did what great managers do, create an environment where we could work as efficiently as we could. If we said it takes X amount of time he wouldn't try to squeeze out a faster deadline, he'd report "it will take X amount of time". If we said it's unreasonably to take feature Y in he'd say we're not going to take feature Y in.

IMO it's much harder with PMs who did some development 20 years ago and "know how things are done". The ones with some technical knowledge almost always butt in.