I don't use Mint, but to me, the main question is whether I perceive the company to have more to lose by abusing the power I grant them than they would gain from doing so. A small fly-by-night company might make a quick buck by embezzling people and then disappearing, but a bigger company has much more value tied up in its brand and customer base, so they would have too much to lose. It's not a bulletproof approach (as some high profile Ponzi schemes have shown) but it's managed to keep me out of trouble so far.