Google AdWords - CPC vs CPA?

Morning!

I was wondering if anyone has used Google's CPA advertising, and if so how you found it compared to the regular CPC method?

From what I can tell the CPA method sounds good in theory, but it looks like you have less control over optimizing your campaign since Google effectively handles more of that. Is that the case? Are there things you can do to better optimize your CPA campaign? Or does Google do a good enough job of doing that automatically?

Thanks,

Thomas.