It didn't just go on sale, it plummeted. Does that work in your favor?Ally Financial (ALLY 0.86%) has become the largest all-digital U.S. bank, attracting customers with its low fees and high savings rates. It's already on many investors' radars because it's a Buffett stock.Its stock tumbled last week after giving investors a disappointing update, and it might take some time to recover. Should you stay away, or is this an opportunity to buy on the dip?All about AllyAlly's roots are in auto lending, starting ...