My wife is the oldest of six, and after number six popped out, he decided to go through with the proceedure.
According to her, he's a big baby when it comes to medical stuff, but after it was done, his comment was, "if I had known it was this easy, I would have done it a long time ago."
Of course I'm sure the youngest of her siblings are happy that he waited to have it done.
As an alternative though (not to try to sway you)...
My wife had always hinted that I might have to go through with this someday. Of course we haven't had kids yet, so it isn't an issue anytime soon anyway.
She recently watched a special on TV where a family had 14 kids. They originally had thought about some form of prevention. Then they came to a conclusion that they trust God in every area of their lives except for family planning. Now they trust in him and are happily blessed with a large family.
I'm not sure how many kids we'll end up having. We'd always said 2-4. Now it's just up to God.
Of course this could take me to a prayer request which should probably go elsewhere, but we're really hoping and praying for number one right now. (especially after last summer's prayer request)