I love this app. Plain and simple. I have always been quick to look up definitions almost on a daily basis as I hear and use words, and Word Vault is the spoon that feeds the vocabulary hungry beast within.
Pros:
-Pre-loads words to make looking something up a breeze
-Home page of each word has definitions, synonyms, antonyms, and an audio example.
-Super easy to save my favorite words, and once saved I can ask the app to quiz me on all or specific chunks of words each day/week/month
-Allows me to copy/paste any word/definition I find elsewhere. I find this to be particularly helpful because I use the Merriam-Webster Word of the Day Widget and need a place to store words and definitions for future reference. This feature also has a dedicated entry spot for the source of any word I save (e.g. Merriam-Webster, talking with a colleague, movie, etc.)
I definitely rate Word Vault 5 stars with no hesitation. I think that anyone who is interested in growing their personal vocabulary base will love this app.
TWF13 about Word Vault: Vocabulary Builder, v2.0.2