I use QuickbooksPro. It is pretty easy to do billing with it. I bill once a month.
My biggest grip with QuickbooksPro is the advertisements for online services that appear at the bottom of every screen. I can understand advertisements on a program that you got for free, but I PAID for this program, along with regular updates. From what I understand, it is basically impossible to get rid of the ads. If anyone here knows how to get rid of them, I would love to hear about it.