I saw the site a few days ago when @mike9r talked about it on Twitter and I am impressed with it.

I have tried Blinksale and FreshBooks (who I guess are your two most obvious competitors) and neither of those really worked out for me, so I am anxious to give Ballpark beyond clicking around a bit. I am constantly looking for something that is only as powerful as I need and fits nicely into my work flow.

My favorite feature is definitely the "news feed" that shows comments / payments / etc. - that is a great way to visualize what has gone on with the project.

And yes, great design :)