While the other engines are good and portable, trying to integrate some native stuffs is really a pain and not so straight forward as the native one. For example, Game Center, OpenFeint integration, photo libraries, camera, etc..
With monkey it's no problem, because it does translate/compile into a real XCode project - so you have a native project (you can also wrap it of course).
You might also have a look at Corona
for iOS / Android. Everthing is written in LUA - it's very easy and has a big and easy-to-use api (I don't like LUA that much, because it isn't type-safe, but I know some developers who LOVES it and they have done serveral cool games with it).