Holy crap that's super useful. For whatever reason I never could figure out (or remember) a shortcut for the 'filldown button'. Instead I would copy the cell I wanted, and then control+arrow my way to the fill destination, shift+control+arrow my way back, and then paste.
This way is a lot simpler and should cause fewer mistakes.
Handy in conjunction with End,<arrow key>, which offhand (I don't have Excel on this PC) does something like: find the next entirely blank row/column in that direction, then extend selection to it (exclusive). If you've got gaps you can just repeat it.
So, if you've got one column filled in for every row, and one row filled in for every column, you can quickly select the whole box and fill it down from the first row.
You don't need to press end to enter end mode - holding down ctrl toggles it. So you can hold down ctrl and use arrow keys and, like it does with text, it will jump entire logical blocks - either continuous cells with content or continuous empty cells. Add shift as well as you select all that stuff instead of just bouncing across it.
This way is a lot simpler and should cause fewer mistakes.