It does'nt really matter if you get a used or new copy,this lens is outstanding!,i have this lens for bird photography,its sharp as!
I see one POTN'er says its an old design bla-bla,i have heard the comment before,well i think canon know it such a good lens for price ther is no need
to change design etc,i think that its "old" and continually made says something in its favour.there is slight CA when pic taken against bright sky.
The only way canon could improve this lens is to add flourite to lenses,but
be prepared to pay,I bought mine new,last sep09,i think someone makes a
good comment if you buy new,you will get a very current one,I think if you are serious about animal/bird photography-just buy it new,of course if you can find a very good copy locally,that you can handle before use,go for it.
Check Fred miranda review,also bird forum,have fun with that lovely lens