I might surprise you here.
I was very much against it and then I investigated the facts and had a change of mind
You may be surprised that it's not 100% effective but it's in the 90-100% range
http://www.jaapl.org/cgi/content/full/33/1/16/ T2
The issue is in forcible castration - this should only be done voluntarilly - you might think who would but a lot of sex offenders know they have a problem and a surprising number will especially if there's a reduction in sentence involved.
There's no place for it if it's compusory or if undue pressure is brought to bear but otherwise it could be a big step forward in some cases