Just bought this for my treo 750 (windows mobile) today.  The dictionary is stellar.  Everything you need from antonyms, synonyms, and etymologies.

Don't put a lot of stock in the "speaking" part of it though.  The speech engine is far less than perfect.  But that's the case with the other two dictionaries I tried as well.

But if you just need a convenient, carry around dictionary that's about as thorough as the OED, this is the one.  Note that a comparable dictionary in print would run about $80 more.