I have four different Thai dictionary apps on my iPad and this is the one I bought last because it was most expensive. Save yourself the trouble and buy this one FIRST. Its so far above-and-beyond the competition its not even in the same league. I cannot imagine studying Thai without this app. The fact that it clearly marks which words are "formal", which words are "common" and which words are rude is invaluable. The "Find words inside" feature is also extremely helpful for understanding compound words and remembering what they mean. I could go on and on. If you want top of the line, this is it.
Obiofulious about Talking Thai <> English Dictionary+Phrasebook