I tried out these services, and it just freaks me out too much having $50,000 sitting in an iPhone app. I get that they are insured and legit, but it's just too much money for me to hand over to a startup.
Every single one of my brokerage accounts and banking accounts has an iPhone app. You probably know this, but the money doesn't actually "[sit] in the app", whatever that would mean. It is invested in stocks, bonds, mutual funds, etc -- the app simply gives you a view into where it all is and the current value. :)