I've gone through testing with a handful of these JS to Obj-C conversion systems and haven't found one that does a really good job of emulating a native app. Appcelerator seems to be the best option out there, but you write very Appcelerator specific JS to make things work.

While quite straightforward to work with, my main gripe with PhoneGap and QuickConnect (a similar system to PhoneGap) are: 1) the inability to have a native footer (a fixed header is possible, just not the footer)

2) no keyboard views other than the standard (eg: no number pad)

Appcelerator gets you access to both of these things because it's actually re-writing your code as native (at least that's my understanding).

Basically, for side-project or pretty simple apps, PhoneGap seems like a godsend. But for a commercial app that needs a certain polish and a truly native feel, not sure they cut it. My 2.5¢.

Also, I did a quick write up on this back in June: http://DesignLitm.us/u/14 If things have changed since then, please ignore this entire response!