Firstly: Like all other code obfuscators out there (that don't require you to install server-side extensions), this method of obfuscation is not 100% foolproof. Any code that can be eval'd can be read. But although possible, being able to read the original code would be hard and take a lot of effort. Unlike other PHP code obfuscators, this can not be decoded/decrypted by the obfuscation decrypters you can find online.
Protip: The "Encode variable names" and "Encode user-defined function names" obfuscation options might not be compatible with your code, so if your obfuscated code doesn't work then try disabling these options.
And finally... I provide this as a free service and would like feedback on this, so if you have any suggestions, bugs or just want to say thanks, let me know in the comments form.