I am in the beta program. It has a certain promise to the interface and the team is very responsive when it comes to bugs. However, there are frequent and sometimes significant bugs that have really made it impossible to do serious development. Everything is essentially in a webview with no low level socket connections and only limited native controls. All this being said, it's a lot better than phonegap and will likely win out in the long run.

Here is another potential one that is in beta. Ansca's development environment is based on the Lua game programming environment and compiles to native iPhone. Have not yet had time to check it out yet but it seems to be very promising.

http://www.anscamobile.com/corona/