I haven't used it, but my biggest reason for not using it is that there's a possibility that Apple could at any time remove all apps made with MonoTouch as it seems to go against the developer agreement:
Applications may only use Documented APIs in the manner prescribed by Apple and must not use or call any private APIs. Applications must be originally written in Objective-C, C, C++, or JavaScript as executed by the iPhone OS WebKit engine, and only code written in C, C++, and Objective-C may compile and directly link against the Documented APIs (e.g., Applications that link to Documented APIs through an intermediary translation or compatibility layer or tool are prohibited).
Having said that though, Unity licenses MonoTouch and they seem to have a decent relationship with Apple.