a couple generalizations from someone who has NEVER worked for AMR:
AMR is a for profit company, meaning they need to stay in business by answering as many calls with as few resources as possible. this leads to high call volumes per unit, and overworked employees, and high burnout rates.
pay rates tend to suck, since company profit comes above employee pay.
ambulances tend not to have the best stuff, nor are they always over equiped. again, less equipment on the ambulance translates into more money for the bottom line.
the company only makes money if they transport, so they encouraged to transport EVERYONE.
Some more generalizations:
private companies tend to be jobs, not careers. high turnover, burnout, low wages, all lead people to go elsewhere. the upside to these agencies is that you can gain experience, your coworkers will be your future connections when you want to move onto bigger and better things, and odds are, most people in the area started out working for some kinda of private company in before they moved onto bigger and better things.
plus, they tend to be easy to get into (generally, not always), so if you need a per diem job, and some extra hours during your downtime, they are almost always hiring.