"Effective enough" depends entirely on how effective you need it to be. And that depends on what you are protecting, and from whom. None of the conventional methods (obfuscation, encrypting the bytecodes, compiling to an "exe") will stop a skilled and determined attacker with enough time and incentive. But that pretty much applies to all forms of programming. (You can disassemble or decompile C/C++ apps as well ...)
The only way you can protect against a serious reverse engineering effort is to use a secure execution platform; e.g. using something based on TPM. Even then, if the bad guys can attach a logic analyser to a system running your code, they can (in theory) capture the native code being executed and then start on the reverse engineering path.
EDIT : Someone has reportedly succeeded in breaking a popular TPM chip, using an electron microscope; see this Register article. And interestingly, his original motivation was to hack Xbox 360 consoles!
