This is quite an effective budget shower for use while camping, especially wild-camping, in the summer.
I pop mine on my car roof during the daytime, leaving it in the sun for as long as possible to warm up, then return to it for a quick shower in the early evening. By this time, I'm hot and sweaty and more than ready for a shower - so this is just what I need and I leave it on the roof with the shower hose directed between the opening rear doors of my Kangoo. Obviously, the shower doesn't have a thermostat and the temperature it reaches is governed by how sunny the weather is and the outdoor temperature but on a warm day (25 deg) it certainly warms up the water.
I find it works best by not fully filling the bag - which helps ensure a more even water temperature and also makes it easier to lift as 20 litres weigh 20kg and this can be quite unwieldy to lift above head height.
The only drawback is that, while it's good for a warm trickle shower, it is useless for washing long, thick hair so this is something I have to forgo but - hey, I am camping and there are ample camp-sites available for anyone wanting full facilities and all mod cons!
If you're looking for a means of having a warm rinse-down at the end of the day and want to avoid camp-sites, then this is just what you need.