Mint is an Intuit company (makers of TurboTax and Quicken), so you know it will be safe (basically Mint isn't hosted by Aljabej Onhandra who lives in Nigeria and wants to help make you rich). After creating an account on Mint you can easily sync up your bank accounts and then every future transaction that you make will be recorded on this site. They have lots of tools that allow you to see where you're spending the most money which is extremely helpful, and also a little scary (who knew you could spend so much on beer?) Another service that they offer which is nice is that they'll suggest other bank accounts or credit cards that will offer your better interest or more cash back. One thing to note is that their suggestions are based off of these people paying them, so always before you apply for a bank account or credit card make sure to research it (both here and the rest of the web). Overall I've been extremely happy with Mint and have been using it for over 2 years, and still use it to this day.
TLDR: Grade A; Extremely helpful in getting your finances and spending organized. Offer lots of tools to cut down on needless spending.