we bought one form this place
www.birthpoolinabox.co.uk last weekend we did a trial run on it and it was VERY sturdy! i was very surprised for it being a "blow up" pool! i agree that the sides had to be 6 inches in width at least and the plastic is a very good sturdy plastic, not the thin plastic you find on your children's blow up pools...etc. i am completely confident i wil be able to hang over the sides and have MW or dh pushing or hanging off the sides as well and not worry of it bursting.
i have a very large bathtub and the only reason we decided on a birthpool is that my dh could really get in the birthpool with me and be comfortiable, plsu my own options of positions increased dramatically with the birth pool over the tub. if it was just me and only i wanted to catch the baby, i wouldn't mind birthing in a tub, but since dh wants to be so involved the birth pool was a much beter choice for us. (and i didn't realize this until we set it up, but the birth pool has a 3 inch thick "bubble floor" making it very comfortiable and cushy! so when i'm on hands and knees it is GREAT cushioning from the floor underneath! i don't have to worry of putting comforters under the pool to cushion my knees for me!)