I haven't seen these at my supply house, but I
just saw them online on the manufacturer's website.

They are "old or new work boxes" that are like the
SmartBox or Arlington OneBox in that they have
internal screws to attach to a stud.

It appears that the difference is they have a
slider plate that allows you to adjust the box
depth after it's installed. This seems like
a pretty good idea.

I usually stock OneBoxes, but was thinking of
trying out these "SliderBoxes", and was
wondering if anyone had used them and if they
liked them.

Thanks.

I am getting my supplier to bring some in so I can see what they are like

One of my suppliers started stocking them instead of the Smart boxes last year, so I decided to try them.
They are definitely a big improvement over the standard Smart boxes and I think they are even a little less expensive for the one, two and three gang sizes. I haven't used the OneBox though, so couldn't compare them to Arlington at the moment.
Either way, I'm sure a lot of box extensions and 1-1/2" 6/32 screws could be saved by using these boxes in walls that will have a tile backsplash installed.
I see that Allied even makes round slider ceiling boxes, which look interesting, but I haven't had the opportunity to try one of those yet.

SliderBox