I had it for a while (actually I still have it, but it's broken now ).
It does improve dustcollection significantly for regular cuts (it doesn't make it better on those cuts where you trim off just a little).
You can't use it in conjunction with the green splinterguard, and you can't use it for bevelcuts.
It does block the view of the sawblade, so it makes stopped plungecuts harder.
I use my ts55 onsite most of the time, and the inlay of the systainer is shaped so you can leave the thing on the saw.
I was making a cutout in the middle of a piece outside at a clients house with limited space in the backyard. Someone [embarassed] must have stepped on it because one of the bars wasn't parallel to the other anymore, when I bent it back it came off. (hence the broken-ness [wink])
The other use for it, aside from improving dustcollection, is making shadowcuts, for instance cutting flooring that's already in place parallel to the wall.