I would like to pick up this snippet from a recent POD Critic post: "This site does not attempt to be a workshop; I operate a little farther down the pipeline than that. Many of the books that pass through me, however, would benefit a great deal from being workshopped."
I must say that most of the self-POD books I have read so far have had noticeably more errors than comparable mainstream novels. However, I can only think of one where the errors were frequent and severe. Nevertheless almost all writers do need get impartial outside input on their work before either self-publishing or submitting a manuscript.
By far the simplest way to acheive this is to exchange critiques with other writers. In my opinion it is best when these critique partners are not personal friends, when they are at your skill level or above, and writing in your genre. You can contact critique partners through writers forums (such as the Absolute Write Water Cooler or Romance Divas) and egroups (such as yahoogroups). But, failing that, it can be very valuable to take part in critique exchange sites such as critiquecircle.com and critters.org.
[ETA] See also this from PODling Master.