They're meant to work by toning the uterine muscle with the idea that labour is 'easier' as the muscle has been 'exercised' and is prepared. Raspberry leaf is also meant to help get your uterus back into shape after the birth.
I felt bh and had major cramping after 3 cups of the tea but then had nothing else, the bh/cramping is normal because thats really what the tea is supposed to do - work that muscle!
I think it's one of those things that if you really think it will work well for you, then it will otherwise you may as well drink water. A lot of it is the power of your mind.
Take it if you think it works for you! you're not really going to know if it makes the birth easier or not, are you? Unless you were in a parallel universe and could see what you would've been like had you not taken it...but that's another story for another day