From what I hear, it can be awful, especially in California. I've heard of stories where people had dozens of years teaching experience, a PHD in their field, and they couldn't get a job at a high school (they wanted to get into HS teaching... they didnt HAVE to get into it :P) because they weren't credentialed. Oh and as someone pointed out, they don't care if you know your subject. I remember reading an article about a test school they were trying out in new york where the pay was extremely high so when teachers were interviewed for the job, they were actually grilled on their subject and ability to teach it and some who were interviewed (that is, by a newspaper) said it was the first time in their 30 years of teaching that they were actually asked about their knowledge of their subject.
Everyone I know and people who have known peers who went through credentialing said it was the biggest waste of 2 years in their lives, yet everyone knows you have to do it, no exceptions.