From all the app code generators I've seen and used, the experience is absolutely horrendous. Memory leaks, does not five you full access into the API etc... It's a lot easier to suck it up and learn Objective-C than having to deal with these tools.
My reservation is the cost. I bought MonoTouch when it was first released and wrote a game for the iPad with it[1]. If I wanted to release that game now though, I'd have to re-buy MonoTouch, which doesn't sit well with me. But if you're not a hobbyest then the cost is pretty irrelevant IMO.