I've been worrying about the same thing - productivity in C# instead of playing with an unfamiliar language, vs developing how the OS developers intended it to be.
Its easy to list pros vs cons -
pros (of monodroid, for C# developers)
- familiar dev environment, can develop using Visual Studio
- C# instead of Java
- Reusable code assets - you can use a lot of code in your iphone project or Windows phone 7 project using monotouch and the WP7 SDK.
cons -
- debugging could be difficult, due to lack of api documentation from Google. Will have to depend solely on monodroid documentation.
- there could be a lag when it comes to latest cutting edge features - safe to assume that the monodroid team will require some time to wrap around the latest android features every time there is a new OS release
- availability of libraries - I am not sure how you can use the open source libraries that would have evolved around the android platform, that could cut down on development time