More and more IEP's are diversifying into the university as a whole which may make it more likely to need a PhD in the future. Most jobs on a temporary basis. Often required to teach from a set curriculum. But most positions part-time except for coordinators. Can pay quite poorly but seems to epend on state.
Jobs can vary greatly.
But too many ESL positions paid at aid pay or done in some other questionable way to defray costs. Thus, requirements can vary greatly to to ability to speak English to a PhD in Linguistics. Looks good on a resume and usually an invaluable experience but can be difficult to be available for jobs in the States. Some people have very difficult time readjusting to life in States also. In sum, you are unlikely to get rich teaching ESL but will quite possibly be immensely satisfied with what you do everyday.