I've been using the YNAB[1] app for expense tracking, and I'm pretty happy with it.

It has a nice feature where it records the retailer's location using the iPhone's location services, so when you enter new transactions for a retailer you've been to before it can automatically guess the payee and category. After a week or two, the app had learnt the payees I used the most, so entering transactions became super fast.

The downside with YNAB is that you must sync it with a desktop version of their app. The iOS/Android apps are free, but only support a small subset of the desktop app's functionality.

With YNAB there's also a whole financial management system that they want you to buy into. It works well for me, but might not meet everyone's mental model of how personal finance should work.

[1]: http://www.ynab.com/