Mint only works, perfectly and automatically, if all your income and expenditures are done through banking institutions. In my case, all my sources of income come from automatic deposits to a bank account, and all my expenses regardless of the amount, are done through pre-authorized debits, credit card payments, or cheques. Rarely do I have to do manual adjustments to where the expense should be allocated, but it happens once in a while. All my investments, including RRSPs and mutual funds, are also tracked on Mint. Mint provides me with a quick and precise snapshot of my financial situation.
Edit: as Zikoris said below, I also do not use Mint to budget. My budgeting principles are essentially like his. No to a bunch of useless expenses (useless to me, maybe not to others: cable TV, toys of all sorts, etc.), and yes to things that I consider either needs, or lifestyle enhancers (travel, art shows, quality time with wife and friends, good quality clothing, etc.).