Yeah, I've used them.
First of all, I'd recommend you buy the cheapest you can get - often there are two-for-one offers, etc. Our group used a big mix of makes and none were in anyway superior to any of the others that I could tell. Also, realistically unless you're lucky enough to be able to go on lots of skiing holidays, you're not really going to get that much use out of them, so there's no real point in paying extra. They all adhere to some EU radio standard so as far as I can could make out all have the same range (presumably the regulation limits their power output) and support the same set of frequencies.
Secondly, the results have been mixed. The first year we got 'em we seemed to be amost the only people in the resort with them so they were great. Since they've become more popular, it's more difficult to find a free band/sub-band. One year, it seemed you could contact anyone almost anywhere on the mountain but last year at a different resort (due to the shape of the mountain, I guess), the results were more mixed but they were still good for finding out where people were in the village, etc.
More and more people seem to use their mobile phones as coverage on the mountains has been improved. Despite the very high roaming charges, this might be a better option if you can keep the length of your calls down. You'll probably want your mobile with you anyway as a last resort.
They are great when they're working because you don't have to arrange everything in advance and you can split up into smaller groups and still meet up for lunch or beers or whatever.
