I thought I'd download this and give it a shot as well. I had tried a program called 'Time Trax' (I think that is what it was called?) but it wasn't nearly flexible enough for the way I get things done in my office. Will let you know if there are any valuable comparisons to be made.
I too bill at 15 minutes. I also require a minimum for some types of projects - because some things may take literally 10 minutes but are very labor intensive. Thanks for the link!