This is an ingenious product that allows much more efficient use of shelf space but there are a couple of things to note if you're planning on using it as a spice rack: 
1) The top shelf is not as tall as the bottom shelf. It's not clear from the pictures of the product, but standard 4.25" spice jars will not fit. Jars that are 3.5" tall are a perfect fit. The picture on the box shows two short jars stacked on top of each other, but that's quite a tight fit and you'd have to remove an adjacent jar to get them out. Really 3.5" is the max height for the top shelf. Four standard-width spice jars fit per shelf so each unit holds 8 total. 
2) In planning how many you will use per shelf/where you will put it on a shelf, remember that you have to leave room for the rack to turn as you pull it out.
Love this product and am very happy to have it, but it would be nice if either the shelves were adjustable or if the entire unit were taller to have equal-sized shelfs.