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.