I purchased a Thai map for my European Garmin Nüvi, 5 years ago and used it in both continents.
Last year I took my Nüvi to the Garmin Service center on Sathon road in Bangkok, to have the latest map installed, because over the time, several roads were added or modified .
I have been driving from Ban Krut to Hua Hin, Bangkok, Ranong and Chiang Mai and found my destinations easily.