I presume you are talking about monotouch or monodroid?
I haven't used those products specifically, but Miguel and his team are first rate and the approach they take is amazing -- it is really a native app on native levels and not something like phonegap. I'd use their stuff without hesitation.
That said, I probably wouldn't use it in anger unless I had some good background in building truly native apps with the native toolkit. Mono et al are solid but there is still a translation layer so you are going to need to understand the underlying code.
I've heard several positive feedbacks about it. Xamarin gives an access to platform’s native SDK and UI controls, projecting the entire native API of each device into C#. Native compilation provides users with excellent app performance for even the most demanding scenarios. Moreover, with Xamarin you can share the code between platforms. Here is the discussion I've faced on Qoura http://www.quora.com/Xamarin Probably, you may find there some helpful tips.