What’s a quick way to comment/uncomment lines in Vim?

I have a Ruby code file open in vi, there are lines commented out with #:

class Search < ActiveRecord::Migration
  def self.up
    # create_table :searches do |t|
    #   t.integer :user_id
    #   t.string :name
    #   t.string :all_of
    #   t.string :any_of
    #   t.string :none_of
    #   t.string :exact_phrase
    # 
    #   t.timestamps
    # end
  end

  def self.down
    # drop_table :searches
  end
end

Say I want to uncomment all the lines in the first def ... end section. What’s an efficient way to do that in Vim?

In general, I’m looking for an easy and fluid way to comment and uncomment lines. Here I’m dealing with Ruby code, but it could be JavaScript (//) or Haml (-#).