After using [url=www.freshbooks.com]Freshbooks[/url] for about a year now, it would be very difficult to sway me toward something else. It's yet another cloud service, and there's a monthly fee, but I've definitely found it to be worth the investment.

I'm not very familiar with invoicing software, though. I never really looked for it when I was deciding which direction to take with my invoicing. Plus, now that I'm switching back and forth between Windows and OSX, it's nice to not be tied to a piece of installed software.