Pity if it's restricted to direct solar water heating, IMHO. We use our PV system to heat our hot water for most of the time, and although, in theory, PV is less efficient, in practice it's a bit cheaper for a given amount of hot water. Most of that is because the price of a PV system of a given power output is somewhat lower than that of a direct solar thermal system, although it does take up a lot more area.
In terms of reliability, then PV also seems to win out for heating hot water. Our neighbour has an evacuated tube solar hot water system, and she's needed to have it maintained regularly in order to keep it going, and even then it's packed up on her a few times. It has meant that I've found out a bit about how they work, though, and how to fix them when they don't. It seems there aren't that many people around who understand how solar thermal systems work, although she's found there are lots of them that want to sell her a new system . . .
