I haven't used an entire rack, per se, and definitely nothing for storage, but I have built a few entertainment centers using the vertical rack rails set up for the industry standard width shelves as well as custom shelf widths.
They'll hold some heavy *** receivers/amplifiers on each shelf, so weight capacity shouldn't be an issue as long as you use 3U-4U shelves. There are also a few companies that make drawers and stuff with the standard width as well.
IMO, there are a few drawbacks to using them for storage
1) that standard width. 19 1/8" if I recall.
2) if you put several racks side by side, there will be a few inches between the shelves in one rack and the shelves in the next rack. Good for air circulation, not so good for capacity.
3) the shelves will have vertical flanges on each side running front to back to support the shelf itself. Not so good if you want to try to use shelves in neighboring racks to store long stuff, although I suppose you could flip the shelf over.
If you've got a bunch of relatively small stuff, it should be perfect.