Occasionally doesn't work with some words if the selected word isn't in all lower caps (example word: Substantially). Maybe an issue with Wordnik itself, as searching for "Substantially" (with caps S) will result in no match, and a suggestion link to "substantially" (with lower S). Probably can be been solved if the add-on internally converts the selected word to lower case (probably, since I'm not sure if Wordnik has some words where they have different meanings depending on case).

Other than that, the orange bottom popup thing doesn't work that great on some sites if they have some sort of widgets on the bottom area that have pretty high z-index. A popup below the selected text would've worked better.

Very simple and straightforward to use.