They installed me in the foothills of Etiwanda (Rancho Cucamonga) with the 18-inch dish and it works fine, even in the rain and wind. Only twice during that really big rain and snow storm we got about 10 days ago did I lose the signal, but only for a moment. I was paranoid about the 18-inch dish as well, but it seems that their software updates over the last few months have helped reception. This last update a few days ago completely cleared up the pixelation that occured when I rapidly changed channels. Very heavy rain will SLIGHTLY degrade the picture, but it does that on my Dish system as well. Light to moderate rain is not a problem. My off-air stuff is a joke, but that's not VOOM's fault -- it's my location. Besides, it looks as thought HD locals will be carried by VOOM and their competitors at some point in 2005 which will render the OTA issue moot.
Good luck with your install. My fellow was here for 3.5 hours and did a good job.