In reply to girlymonkey:
They are not really an alternative to water, ideally you use both together:
Water is much more cooling, and should always be used first (preferably in combination with a cellophane cover as this gives an excellent analgesic effect). The water cools the tissues down so the ongoing burning to the skin stops. Also water is useful for chemical burns as it will wash chemicals away / dilute the effect which dressings will not.
Burn dressings are good after 10 minutes of cooling the burn down because they keep the area wet (and covered to help prevent infection). Of course if you have no source of water the burn dressings are better than nothing (or just using cellophane) because there will be some cooling effect of the dressing.
I don't know of any evidence of a difference between gels and dressings. The idea is to keep the area moist and clean and I suspect both do this in a similar way.
My personal preference would be to not waste money on them unless expecting prolonged transport times to definitive healthcare, or working in an environment where you are likely to see a lot of burns. I can't imagine any of the mountain rescue people on here carry them but I would be interested how they rationalise the weight and cost if they do?