I live in northeast Thailand, far from Bangkok. When I speak Thai up here, I get no sense that anyone is offended. I often have to repeat words, mainly because although they sound okay to me, I'm using the wrong tone (high, low, rising, falling). Other than that, I get along fine up here.
When I go to Bangkok, however, I get a lot of the opposite. When I'm in a store and try to talk to a Thai salesperson in Thai, they often seem offended, or at least bored, and start talking to me in English. Once, a particularly offensive Thai just flat-out replied to me in English, "You don't speak Thai good. I can't understand you. Can you speak English?" To which I replied (in my broken Thai), "Kuhn mai phut Anglit dee. Pom mai khao jai. Kuhn phut Thai, mai (with an accentuated rising tone).