Andy,
Lived in Thailand for three years and loved every minute of it, if your mind is open enough, you will too!!
For a month, I would definately reccommend a combo of city / beach / mountain as others have suggested.
Personally I love Bangkok but others can't wait to get out of it. Great for shopping, eating etc.
Ko Samet is a great island for a get away although slightly limited for decent accommodation.
Chiang Mai is a cool easy going city in the north and a good starting place for treking in the hills, etc.
The Thai people are generally amazing. Yes someone will try to rip you off at some point but that happens everywhere!