We had this year at Rock am Ring (2013) all 4 of these showers for 7 people in insertion!
We were very satisfied! Although it takes quite a few hours until the water is pleasantly warm, but it's very enjoyable!
Tip 1: take Mehrer showers, so you can still hold enough warm water! (Is very convenient, we have already exchanged the odd warm shower to cool beer;)
Tip 2: If a tree or the like in the vicinity, you should ask someone if he keeps the shower (and the shower head)!
They can indeed lay it on the roof of a portaloos, but this is impractical and ... not soo great.
