Not exactly, but I have done the home-made equivalent (fit an external PHE to an existing DHW tank). This is an awful lot cheaper and seems to work pretty well. See http://wookware.org/solar/ and http://wookware.org/pics/solar/036-IMG_2548.JPG.html for a pic of the PHE in place before the rest of the plumbing and insulation was done.

I've been very impressed how well the thermosiphon works - it responds very quickly (< 30 seconds), although I reckon there is some reverse circulation occurring from time to time, which reduces efficiency. Last weekend was the hottest day of the year so far at 87C: http://wookware.org/solar/solardetail1.png

Doing things this way lets you fit a solar system very cheaply as it doesn't require a new cylinder. The solar gain is not as high as with a standard twin-coil cyclinder, but the solar fraction may well be higher, because hot water is delivered directly at the top of the cylinder (i.e. this approaches maximises usable hot water with no boiler assistance, not total energy gain, following the idea that 50l of actual hot water is more use than 120l of tepid water).

Unfortunately I have no way to directly compare with how the equivalent twin-coil system would have performed, but I can certainly say that this scheme has kept our boiler off since April. There have been a number of days when the tank was not really hot enough for a bath, but as you can see from the pics the tank insulation is currently terrible. When that gets fixed I expect there would have been very few days in that category.

Last week's tank temps (in generally mediocre weather): http://wookware.dyndns.org/munin/localdomain/localhost.localdomain.html