For the past several years, I have used Thai Visa Centre exclusively for all my immigration needs. I have a Non-O visa for the purposes of retirement and while I could do the extensions myself, I find that using Thai Visa has several advantages. First, I don't have to deal with immigration -- ever. I haven't been in an immigration office since I started using them. Second, they can handle all of my immigration needs: visa extensions, 90 day reports, residence certificates, visa transfer from an old to a new passport, etc. All of this can be done online and through the Thai Postal EMS service, which is secure and reliable. In Bangkok, they will even dispatch a messanger to pick up and return your documents. Of course, it can be nerve-wracking to send off important documents like a passport but they have been very responsive in ensuring that my mind is at ease and to answer any questions I might have. To me, the cost has been well worth it for the peace of mind they offer.