I basically agree with lulu, who does after all know what she is talking about here!
BUT
I am not sure if it is always a good thing to have your uterus strengthened
I have had two very short active stages (1 hour). Both times were also very, very painful, which I think was partly because I really had no time at all to acclimatise to increasing contractions. I also had to transfer from a homebirth due to blood loss, which the mw put down in part to the tea (am not sure about this really, have not found much to back this theory up).
I also had, both times, long painful passive (?) stages.
I know this might just be me and how I give birth, but I have done a bit of research and this time, for me, I am inclined to give it a miss.
Am absolutely not trying to put you off, or scaremonger . I think its a tricky one. From doing a fair bit of reading, I think I probably am inclined towards quick labours anyway, and this just tips it too far IYSWIM. The other side to it is that I have done NCT stuff and I know a lot of women take it and swear by it.
I think if I were a first timer, I might try it and see how it went. Part of my problem may have been that I rather liked it and so may have o/d a bit