I'm doing it now for my first Android app and the JNI isn't as bad as I expected. I'm using juce and it has a bunch of macros that reduce the JNI calls down to 1 line, and there are a classes to convert the types. The UI is all in a webview anyway. It took me under two weeks to learn the Android tools and get a pretty significant app ported.

But I was horrified at how bad the tools and emulator are. I spent more time trying to get things to work rather than porting code.