Take a look at the FAQ, I think your questions are answered there.
http://erikhinterbichler.com/software/herald/#faq
The reason Mail comes to the front is because it's hidden. If you close the Mail window or minimize it then this won't happen. This is a limitation with the Mail plugin system that's very hard to fix, though I will try to find a solution in the future. If your workflow requires hiding Mail then I agree that Herald probably won't work for you. But for people who just close or minimize Mail, then it will work fine.