[yamalight]

well, monotouch is a really nice framework. but, speaking about downsides, you'll need to spend time on writing custom objc bindings if you would want to use something that is not bound yet (e.g. custom library). and that can take quite some time. other than that - it's quite the same as native (may be a tiny-tiny bit slower)

[fajran]

This. I am currently using Mono for Android, also from Xamarin. I use it because the client wants me to use it instead using Java. Like what yiu said, one thing that I dont really like about using Mono for Android is I cant just use Android libraries out there since I need to create the binding first. Other than that, it's just a matter of language preference.

[crisedward]

so, if it's for normal development it should be fine, but if need to use libraries, plugins and stuff for a more deep app I should learn objective-c?

[yamalight]

well, for normal dev you would also need to know the basic iOS SDK classes, structure and their usage. the main advantage is all the c# features and c# syntax (i personally don't like objc at all) you can use in iOS dev with monotouch.