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From: http://stackoverflow.com/questions/1676632/whats-a-quick-way-to-comment-uncomment-lines-in-vim
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: First, go to the first line you want to comment, press Ctrl``V, and select until the last line. Second, press Shift``I``#``Esc (then give it a second), and it will insert a # character on all selected lines. For the stripped-down version of vim shipped with debian/ubuntu by default, type : s/^/# in the second step instead.