Review: I have two of these. One really saved me on a job yesterday.
Client had a rail-mounted closet system (EZ Closets, Martha Stewart) in a tight walk in closet.
I noticed one shelf was not level, I thought someone else misplaced the pins on one side. Then I realized part of the support rail was not properly installed, and it was pulling away from the wall, and two of the adjacent vertical boards were sagging about an inch.
I used one of these to jack up the vertical panels, (they don't touch the floor). I was able to install a 3" x 3" L-bracket under two adjacent verticals, with a very heavy duty drywall toggle.
It would have been very difficult for another person to muscle the shelves up and hold them in place without this jack.
Absent the jack; the closet would have to have been completely emptied out, taken off the wall, and the rail re-installed (easily a half day of work).
Yeah, these things, like most good tools, are worth the price.