We are currently in our 4th KOA as workampers and enjoy it very much. Each opportunity has been from 4 to 7 months long. When it gets cold in the north we head south and when it gets hot in the south we head north. We have a list of must have requirements before we send our resumes to or accept an offer from a kampground. Knowledge of Kamsight and Kampstore is required at many KOA's so one can't be too selective for the first job. However, once you have some experience it is much better.
On the topic of the $35 membership fee; this is a nominal fee which I believe keeps the not-serious from signing up. Once you have a workamper "Star" KOA provides up to 5 free camping certificates for you as you travel from one job to the next. IMO that more than covers the membership fee.
Like any job, KOA workamping is not for everyone. Every KOA is different and some owners are easier to get along with than others. All in all, IMO, if you do your job and mind your own business you will have more good experiences than bad.
Good luck and happy kamping.
Smooth