I found them completely necessary when I was doing NB imaging from inner Sydney. Here are a couple of threads comparing them from when I first got them:
http://www.iceinspace.com.au/forum/s...d.php?t=125922
http://www.iceinspace.com.au/forum/s...d.php?t=126305
The price is now
much higher though due to the dollar (almost double) so I'd have to think harder about buying them now days.