Well, I used an open system (a Libbey) and came out fine. I controlled what to hold in and what to expel - not the therapist. It was cheaper than the closed system as well. I was told (during the session) that the water had to stay between 98 degrees and 110 in order to prevent - I think she said - hypothermia? All I remember it was what Leonardo Dicaprio's character died of in the Titanic.
Also, with the open system, they leave you alone in peace and put on good music, give you rocks, etc. You need to rub your own tummy - but - by far, this worked much better for me than the closed system...