I borrowed one once (there was no way I could afford to buy one - a new saddle was around the corner) and I couldn't find any difference in the going of the horse in it as opposed to a normal £15 bit which looked similar - you can stick a leather curb on pretty much anything anyway, or not, so I think that its really a bit of a waste of money.
I agree with tinks - if you are set on getting one, neue schule is way cheaper!