Ground Instructors
Having been a Ground Instructor at PPL through to ATPL level, I have to say that I think it's been one of the most rewarding aspects of my aviation career so far.
Ok, so you don't clock up those hours as quickly as FI's - which tends to be the argument from other sides, but it didn't stop me from getting a really good job on a great a/c, with a full service carrier. Infact, it really paid dividends when it came to technical interviews & written tests. One company offered me the position purely on the strength of my previous teaching experiences, & from where I had been teaching.
Personally, most of my work was at ATPL level & we had to be asessed by the CAA in order to be compliant with JAA standards, & therefore I am not too sure about the AOPA cert. However, I would still think its a good investment in the long run, if not for the CV alone.
Good luck
HPC