I had one fitted to my O'three suit. Totally reliable but not that useful I found. By the time you get out of the water, get out of your kit, get to the head, unzip, phaff about trying to get through several layers of undersuit to liberate 'mini-me' aim, at the porcelain and fire - you could have unzipped your drysuit in the same time.
I found that as my dives became longer (deco) the zip just wasn't a practical option and I needed a valve fitted. Can't unzip in the water... Well OK you *can*
