Matthew Maddock wrote

We are moving to France in a few months to a house that needs
renovating etc. There is no existing hot water system, so that got me
thinking that instead of putting in the usual electric water heating
system that is common in France, a solar system might be nice.

They're hardly ever good value where mains power is available.

I've seen this system on eBay
http://cgi.ebay.co.uk/ws/eBayISAPI.d...tem=6064859125
can anyone comment on how good these systems
are (not necessarily this one specifically)...

Very over priced basically.

Where we will be, there is plenty of sun, but it also
gets prettycold in winter too (tho still usually sunny!)

... are the systems any good?

They do work, lousy value tho.

will it actually work to a satisfactory degree?

Really depends on how long you go without sun.

or will the electric backup element always be running?

No, not if its sized properly to the use of hot water.

Considering that I can buy a large French "chauffe eau"
(pressurised, highly insulated electric water tank) for
around 200€ max, is it worth it?!!

Nope.