What Sheryl said is true. I went to the Chiang Mai SS facility a month after I left my long time employer (this was about 5 years ago) and was initially told in one office that it was "not possible" and "the policy was changed" and that farang (and presumably all non-Thais) would not be able to get Thai SS after employment. That was very disappointing. I wandered around the building and found another office and asked there. No problem! There was some forms to fill out and they needed various information from me. Took about an hour but it all worked out. A few weeks later I got my blue plastic SS card in the mail. I pay 432 Bt. per month (can pay at any 7-11). In Chiang Mai my assigned SS hospital is Lanna Hospital. In five years I have only used it once for an outpatient procedure with a specialist (I have been very lucky healthwise). Did not cost a single Baht.
For routine tests, consultations, or diagnosis I pay for them myself and go to either Bangkok Hospital or CM Ram. This is because the initial entry process for Lanna is so crowded and annoying. I will use SS when I need in-patient treatment or an operation.