The primary reason I use WordLookup is because it allows me to search EDICT, ENAMDICT, and CEDICT in one program. (ENAMDICT is a specialized dictionary and very huge, and I appreciate being able to separate its results from the general dictionary results of EDICT.) I can search in one language and then copy and paste into the other. When Chinese and Japanese forms are different I use the Unihan Variant Dictionary to generate the variant than I can then paste into the other.
The developer is very responsive; when I requested user-definable key shortcuts to switch between dictionaries, he added them. (Cmd-1, -2 and -3 switch between the three dictionary windows.)
If you ever need to print, WordLookup generates very nice output. Its copy and paste is also very clean, so moving search results into word processors or e-mails is a cinch.
For all of these features and the great support, the $14 registration has been well worth it. I use this program every day.