1. It's secure. From their website under the link to security: "Mint uses the same 128-bit encryption and physical security that banks use. Our practices are monitored and verified by TRUSTe and VeriSign, and supported by RSA Security." It's also a read-only app so while it can see your finances and investments, it can't actually do anything to them (for example, transfer them elsewhere.)
2. It's free. Mint was opened as a personal budgeting tool in 2005 and was acquired by Intuit (the company that makes QuickBooks) two years later. They've partnered with over 1,800 banks and credit institutions to make syncing your information easy. The app has ads and is sponsored by companies that deal with finances so they're able to keep it free.
3. It's helpful. On my Mint app, I have my bank accounts synced along with my house and property value (which is updated via a partnership with zillow.com), my investments (I've synced my Fidelity account for live updates on my stocks and bonds), and my Cadillac (the value of which is updated live via a partnership with Kelley Blue Book.) They've also analyzed my bank transactions and sorted them into categories to prepare monthly budgets that I can modify. While my investments and property had to be input online through mint.com, it still syncs over to the app once they've been added.
Mint has also been featured in the New York Times and Wall Street Journal. I've already found it pretty useful for being able to view all of my accounts in a single app. You can also add a passcode to open the app if you want an extra layer of security but keep in mind that the Mint app can't move any money or property around, it can only view it.
So here's where you can grab the Mint app for free:
Apple App Store