This puzzle is a thermo-sudoku. In addition to the normal sudoku rules about each row, column, and 3×3 box containing all nine unique values, thermo-sudoku adds the requirement that each thermometer must increase in value from its bulb to its other end. (In this case, letters later in the alphabet have higher value, as indicated below the grid.) This particular example should not prove to difficult for anybody who is familiar with basic sudoku. The completed grid looks like this:
Solvers may notice that the smallest letter in each thermometer is given. The letters at the high ends, taken from top to bottom, spell out THERMOMETER, appropriately enough.
Given the speed of some of the solves, the answer to this puzzle may have been a bit too easy to guess, but there is absolutely nothing wrong with getting the solution that way.