I've used it for a year and love it. I don't have a single bad thing to say about them (and I am extremely critical of services like this).
Freshbooks offers me the chance to invoice clients in an extremely simple way without the need to design my own invoicing system (which was my alternative). I was sold not only on the features and functionality, which are all well-detailed here http://www.freshbooks.com/tour.php and don't need repeating.
A few things that sold me:
- I had a free account for months before moving up. In that time, support, via email and Twitter, was totally phenomenal. They never up-sold me, or I believe even knew I wasn't yet a paying client. Once I got a larger account, the support continued with exactly the same professionalism and zeal.
- While designing a version of my web site, I needed a high-resolution version of their logo to fit into my theme. I emailed support asking for one, as a non-paying client, and they emailed me an EPS within hours and further asked if they could do anything to be of help!
- The service itself is flawless, I have never had a serious problem and downtime has always been announced.
- The site works well on my iPad.
- If you want, you can create the invoice online and have Freshbooks mail it to your client for you for an insignificant and extremely reasonable fee.
- Freshbooks gathers statistics on your industry and will create a nifty "report card" on how you compare to others in your field.
And I'll be honest, the fact that it's a Canadian company didn't hurt :)