Checked my bill and the Credit Card exchange rate was always a little better than cash on the day. Not enough to worry about it, but it's still good to know.
When the cash rate was 8625, the c.c. was 8800, and that 150-200 baht difference was consistent as the rate changed eg when the cash rate was 8850, the card was just over 9,000. The ATM rate was also better than cash by about the same margin. As I used the 28 Degree card, there were no other fees, so still better overall than cash.
Even so, I preferred using cash. It was a bit of interest changing cash daily, and tracking the change. Especially as it only went up during my stay ;) .Edited: 9:44 pm, October 16, 2011