Relle,
I have used a birthing stool for my first labour. I personally didn't like it - I needed to be moving. As Kelly says some women find it helpful in second stage - you can maintain an upright position without the hard work on your legs. I have supported women using a birhing stool that have reported it to be fantastic. One gorgeous woman I was with had to be prised off the toilet as she was pushing (obviously a toilet is a similar size and shape to the stool!). So, some women really find it great. Have you thought of using the fit ball? Similar concept - you are upright and you have more mobility than the stool... All fours is a position that many women find themselves gravitating to. Kelly's idea sounds like it could be a good solution.

I am personally an all fours birther!
Good luck Relle