This 4 U rack is as simple and reliable as they come. I have used it in many different situations where it just made more sense to mount devices against the wall on a rack. I have also used it mount 2U rack servers to a wall (you need 2 racks) as well as 2U UPS units. The square holes work perfectly with Dell rail kits. If you are mounting 50+ LB items such as UPS units, I would suggest a 90-degree brace for additional support from your local hardware store just to be safe.
PROS: Easy to use, reliable, strong.
CONS: The screws that go in the sides can easily be lost when unpacking the rack.
This was easy to install and feels very stable. I did have to replace the nails it came with though because they don't seem ideal for this use. Instead I used 14 x 1-1/2 sheet metal screws to mount this to a plywood backing.
PROS: Low cost, very sturdy
CONS: included mounting hardware was not great (but was inexpensive to replace)