There's a thread about in this in Health & Diet section too. I posted my opinion there but I'll stick it in here too....
IMO, I wouldn't go this route. From what I've read, it's not guaranteed effective (it's something like 99.6%), so you may still have to get them castrated the traditional way. The testes remain intact, so there's still a chance of testicular cancer. Plus, it doesn't completely eliminate the testosterone that causes unwanted male behaviors (aggression, marking, roaming, etc). I'd just rather have them neutered the ol' fashioned way and know that I won't have any problems.