I have a Prolite girth sleeve as my horse has a very sensitive girth region and he gets easily bruised by girths. It's a pain in the backside and gets very sweaty and smelly so I'm constantly washing it, but he seems much more comfortable so I guess we're stuck with it!
i use them with certain saddles. they are very good, my saddler says that the results of the latest pressure testing stuff that the WCSaddlers are using, shows that Prolite is much better than sheepskin etc.
if i had a close-contact saddle i would always always use a prolite under it, as many of the pros (e.g. WFP) do.
Prolite does go 'dead' and flat after years of use though, and need replacing.
I use one on Solo's saddle- he's a Tb and is quite narrow.
The saddler checked it before i started to use it though, as it is a temporary measure til i have saved enough to replace his Wintec for a lovely leather saddle.