The certified erectors are always good. The uncertified ones are hit or miss. Some are very good, some are mom and pop (and idiot son), "out of the garage" operations.
The problem is, unless your job is pretty large, it's hard to impose the certification on the General Contractor. The AISC certified shops are typically pretty big, but if your job is small, it's hard to get their attention.
We have it in our specifications, but typically don't enforce it.