well i-mobile 3gx is just a reseller of TOT3G. I dont know why they tell you that its only available in Bangkok, i guess because the SIM packages can only be purchased in Bangkok. On their website they show the full TOT coverage also for their service:
http://www.i-mobile3gx.com/main/3gx_usage.php (scroll down a bit for the maps)
The speed is really good as you already found out. AIS is very over-loaded in urban areas, True is better, but my phone does not support 3G on 850MHz so i couldnt use their card on 3G...