I used Zelix Klassmaster in a commercial application for several years and found it to be excellent. I threw quite a few resources at the obfuscated code, and was not able to "break" it. It's pricey, but good.
I only stopped using it when my version got old enough that the upgrade cost was significant. My needs had changed and I didn't really need to obfuscate the classes anymore. However, if the need arises again, I'd pay for it and use it in a flash.
Cheers,
-Richard