Despite the fact that Stephen Wolfram is British, the app is heavily focused on American English. Although it will provide a definition for colour as well as color, it cant find words rhyming with colour! It would be very helpful if you could select British English as the preferred option.
Otherwise, its delivers lots of useful information for writers including notable uses of words, anagrams and crossword clues, acronyms, words "beginning with", or "ending in" which is really useful if, for example, you want to find all words ending in "mount" or any other word ending.
The app isnt orientation sensitive so it doesnt rotate if you rotate your iOS device. On the iPhone, it only works in portrait mode. Whats hard to understand is why its the opposite on the iPad, where it ONLY works in landscape mode!
For these reasons it loses two stars.