So far, we are very pleased with the PV system. In the first month after installation on 7 Dec 2010 it generated a smidgen over 30kWh. As that period bracketed the shortest day of the year, with the sun at its very lowest, we knew we would not get anywhere near the expected "average" output. As the panels were covered with heavy hoar-frost and/or ice for the first few days, and then had 8 ins of snow dumped on them (which did not clear for several days), I think that output is quite remarkable. Even on cloudy days, the system was still producing power. There were only two days during the first month when zero output was recorded. Both those days had very solid, heavy cloud cover from dawn to dusk - from my flying days, I would guess that each day had thick cloud from just above the ground up to 30 or 40,000 ft. Hardly surprising that no useful radiation reached our roof!
Whereas the highest daily output in the first month was 2.4kWh, we have already had one day of over 3.6kWh since and the system is tending to produce more and more power as the days go by.
Early days yet, but so far it seems to be a viable and trouble-free (fingers crossed!) system.