Merriam-Webster isn't the first dictionary app in the Android Market and it won't be the last. The trouble is that it just doesn't seem to be trying hard enough to differentiate itself.
The Merriam-Webster Dictionary app features: synonyms, antonyms, voice search, example sentences, Word of the Day, and a history of words that you've looked up. That's a solid list of features and certainly the basics of what you need if you want a dictionary app, but it really isn't that much new. Even from a marketing standpoint, at least other apps tout how many words exist in their dictionary (350k from Dictionary.com and 500k from Collins.) Voice search seems to be the feature that is being pushed most by Merriam-Webster, but Android phones have voice input anyway, so that doesn't seem like much of a bonus. Word of the Day is nice, but there is no notification system, so you have to check the app every day.
All that to say, there's nothing inherently bad about the app. There just isn't anything intriguing either. It would be nice to see more OCR options for scanning words with your phone's camera, but the only app we've found that does that is SnaPanda.
Merriam-Webster Dictionary is free in the Android Market, but does contain ads.