gistfile2.txt

                               user     system      total        real
unless                     0.520000   0.150000   0.670000 (  0.677782)
if not                     0.520000   0.150000   0.670000 (  0.672080)
if !                       0.520000   0.180000   0.700000 (  0.691658)

unless-vs-if_not-vs-!.rb

require 'benchmark'

interval = 1_000_000

val = false

Benchmark.bm(25) do |x|
  x.report("unless") { interval.times { true unless val } }
  x.report("if not") { interval.times { true if not val } }
  x.report("if !") { interval.times { true if !val } }
end