When converting code to use symbols instead of strings I got a slowdown.
I thought that was interesting so I thought "perhaps it was the case
statement I added to preserve the interface".  So I tested it.
For this code:

#!/usr/local/bin/ruby -w

class StringOrSymbol

     def with_case(thing)
         case thing
         when String
             thing.downcase.intern
         when Symbol
             thing
         else
             raise TypeError "thing of wrong type"
         end
     end

     def with_if(thing)
         if thing.is_a? String
             thing.downcase.intern
         elsif thing.is_a? Symbol
             thing
         else
             raise TypeError "thing of wrong type"
         end
     end

     def with_if2(thing)
         if String === thing
             thing.downcase.intern
         elsif Symbol === thing
             thing
         else
             raise TypeError "thing of wrong type"
         end
     end

end

wotsit = StringOrSymbol.new()

10000.times { wotsit.with_case("Somethin") }
10000.times { wotsit.with_if("Somethin") }
10000.times { wotsit.with_if2("Somethin") }
10000.times { wotsit.with_case(:somethin) }
10000.times { wotsit.with_if(:somethin) }
10000.times { wotsit.with_if2(:somethin) }

neelix hgs 276 %> 

I got these timings:

ruby -rprofile is_a_vs_case.rb
  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name
 21.47    14.24     14.24    20000     0.71     0.95  StringOrSymbol#with_case
 21.12    28.25     14.01        6  2335.00 11055.00  Fixnum#times
 17.74    40.02     11.77    20000     0.59     0.84  StringOrSymbol#with_if
 17.23    51.45     11.43    20000     0.57     0.83  StringOrSymbol#with_if2
  8.26    56.93      5.48    60000     0.09     0.09  Module#===
  5.26    60.42      3.49    30000     0.12     0.12  String#downcase
  4.69    63.53      3.11    30000     0.10     0.10  String#intern
  4.22    66.33      2.80    30000     0.09     0.09  Kernel.is_a?
  0.00    66.33      0.00        1     0.00     0.00  Class#new
  0.00    66.33      0.00        3     0.00     0.00  Module#method_added
  0.00    66.33      0.00        1     0.00     0.00  Class#inherited
  0.00    66.33      0.00        1     0.00     0.00  Object#initialize
  0.00    66.33      0.00        1     0.00 66330.00  #toplevel
neelix hgs 275 %> 

So it seems for small case statements it is faster to create them
with if...elsif..else...end.  Of course, the case statement is more
readable.  It's ruby 1.6.1 on a Sun Sparc Enterprise 250, Solaris 2.7

	Hugh
	hgs / dmu.ac.uk