Best in its class!
I have been using this excellent on-the-go dictionary app for years on SP and PPC platform.And indeed it is the best among what I have ever used. Now a great news for iPhoners!
One of its great features is that you can find more than you need dictionary library form the internet, i.e. the 203Mb Oxford English Dictionary (OED2), which contains the full 20-volumn paper-based set, and WiKipidia in different languages, and much more!
The author releases updates very frequently. Keep your eyes widely open on it:)
Zhendong XU about
MDict