Its six of one and half a dozen of the other, isn't it..? Soft vs hard. I like the idea of hard cases and being able to open the top, chuck stuff in or grab something with ease, easy to organise your gear and also distibute weight - camping gear one side, personal gear on the other - plus the bonus of extra security. But then there's the option of soft luggage, which is cheaper (to a point) and if the bike falls over, then at least the panniers won't crumple or get a nasty dent. Also, some of them seem to have a slimmer profile - like the enduristan.
I'm kind of keen on hard open-lid panniers and all the benefits they have, and I do like the style of the trekkers but sometimes I think the soft would be maybe a better alternative.