1. Does the app assume that users want to spend about the same amount every day? I'll sometimes spend $0 one day and $1000+ the next day (e.g. mortgage payment). I'm entirely interested in keeping my monthly spending within a limit, but not my daily or weekly spending.
2. I've never taken to Mint or Quickbooks. My finances are pretty complicated, thanks to several income streams and companies, and tools like Mint/Qb assume that you care about ALL of your income and expenses equally. But all I care about is my overall monthly spending (am I going into debt?), and my spending within categories (how much did I blow on eating out this month?).
Have you thought about adding optional categories/tags? So if you want to keep your "beer" spending to, say, $750/month, you can track how much you have left to spend on beer, alongside your tally of total spending? Email/text in "12.55 +beer" and it gets tagged with beer.
The more I think about this, the more I like it. And I love that it's open-sourced. Watch for a fork. ;)