1. implementation

require 'benchmark'

class Object
  def either?(*args)
    args.include? self
  end
  def in?(*args)
    case args.length
    when 0
      false
    when 1
      self == args[0]
    when 2
      self == args[0] || self == args[1]
    else
      args.include?(self)
    end
  end
end

count = 10000000

Benchmark.bmbm do |x|

  x.report "includes?" do
    count.times do
      [:one, :two].include?(:one)
    end
  end

  x.report "either?" do
    count.times do
      :one.either?(:one, :two)
    end
  end

  x.report "compare" do
    count.times do
      :one == :one or :one == :two
    end
  end
end

2. ruby 1.8

1.9.2> ruby benchmark.rb
includes?      1.930000   0.010000   1.940000 (  1.932690)
either?        2.530000   0.000000   2.530000 (  2.533304)
unrolled_in?   3.410000   0.000000   3.410000 (  3.413661)
compare        1.240000   0.000000   1.240000 (  1.236956)
--------------------------------------- total: 9.120000sec

                   user     system      total        real
includes?      1.910000   0.010000   1.920000 (  1.907518)
either?        2.530000   0.000000   2.530000 (  2.525952)
unrolled_in?   3.390000   0.000000   3.390000 (  3.395632)
compare        1.240000   0.000000   1.240000 (  1.238029)