I have been so obsessed with documenting the system performance that my wife has started teasing me about my "affair" with the water heater.  To which I reply "Yeah, once you've had a water heater baby, you never go back.  They're hot!".

But seriously, when I tire of logging data I will need a conveniently located basic system monitor, or a "dashboard" if you will.  So I put a dual display up next to the kitchen sink which is right above the water heater in the basement.  This unit displays the collector temperature (from the pipe as it enters the Solar Wand just above the tank) and the temperature at the top of the tank.  There are 2 lights, a green one indicates the pump circulation, and a flashing red one that warns of low fluid in the overflow reservoir above the collectors.  

August 2009 update:
I have found that the low fluid warning has activated about every 14 months or so.  It is great to have this warning in a highly visible location rather than hidden in the basement utility closet (where I have a duplicate light).  I checked the tank each time the warning came on and it was about 1/2 full and I added 2-3 quarts of 50% glycol solution.  I am able to simply reach out the bedroom window to fill the tank - a nice feature of a wall mounted system.

December 2010 update:
I added real-time web monitoring using an ioBridge web monitor device.  See the LIVE STATS page for current temperatures.

I tucked the collector sensor under a piece of insulation wrapped around the tube that enters the Solar Wand, and the tank sensor I poked deep under the tank insulation at the top of the tank.  All the wires connect to a  terminal strip on the back wall.  The sensors and meters are powered by a wall transformer that provides 12 Volts at 100mA.

At some point in the future I might even build a graphical display that shows the readings for the last 24 hours.