I would say it depends a LOT on who it is. Packt books for instance, are mostly....not very good.
What's worse is that their review process seems to be worse than useless. Recently tried to read one of their books, which credited maybe 10 reviewers, and every other line had me cringing so hard it was unreadable.
I've read an HN thread a few months ago and there was a lot of folks, both readers and writers, complaining about Packt's quality.