We've done this... a few times. The only reason we built our own is because it was five years ago and there weren't very many good alternatives. but now there are tons of alternatives. The main thing we learned in building our own tool is that you will spend a lot of time working on it. And that is time you could be billing for your time. It makes a lot more sense, as a small business, to pay the monthly fee which you can easily recoup with one or two billable hours, than to spend all that time rolling your own. Sure, you'll have to make some concessions, but you'll be far better off in the long run.
As for us, we decided to make our application available for other developers. Check it out at http://www.myintervals.com
