For those tasks I use most of the time block selection.

Put your cursor on the first # character, press Ctrl``V (or Ctrl``Q for gVim), and go down until the last commented line and press x, that will delete all the # characters vertically.

For commenting a block of text is almost the same:

  1. First, go to the first line you want to comment, press Ctrl``V. This will put the editor in the VISUAL BLOCK mode.
  2. Then using the arrow key and select until the last line
  3. Now press Shift``I, which will put the editor in INSERT mode and then press #. This will add a hash to the first line.
  4. Then press Esc (give it a second), and it will insert a # character on all other selected lines.

For the stripped-down version of vim shipped with debian/ubuntu by default, type : s/^/# in the third step instead (any remaining highlighting of the first character of each line can be removed with :nohl).

Here are two small screen recordings for visual reference.

Comment: Comment

Uncomment: Uncomment