The first time I used this product, I had mixed feelings. On the one hand, it is not easy to master and the first time out can be frustrating. On the other hand, it is a unique and helpful tool. If you are too modest to ask your significant other (or your kids - not!) to give that hairy back of yours a buzz cut, you should give this a try. There are several challenges, but all can be overcome with care and practice.
First, read the directions. The main guidance provided is to shave against the way the hair is growing - the "grain" of the hair. Most of your back hair grows down, so that means shaving "up" by extending the arm on the product and pulling it slowly up your back. However, that is not the only direction that your back hair grows. Anyone who has tried this product now knows the several directions that the long-ignored back hair can grow.
Next, don't get hung up on the red splotches that you will see (in the mirror) on your back when you first start using this. They go away real fast. Your back is just not used to coming in contact with something this sharp. And it is not dangerously sharp, just irritating sharp at first.
Further, once you get a feel for this, I recommend NOT looking into the mirror but use this like you would use a back-scrubber or loofah for washing your back -- visualize it on your back and use long "up" strokes. Doing it by "feel" will help keep the cutting edge flush against your skin to get a cleaner cut and avoid the red marks that you make when the cutting edge is slanted against your skin.
Using a hand mirror and the bathroom (or other fixed) mirror is the only way to get started. However, everything goes backwards in the 2-mirror arrangement. If you try to adjust the cutter by looking in the mirror/s as you turn it, you'll likely end up twisting it more the wrong way. Hence the advice to use it like a back brush, don't always be looking in the mirror.
Best to use before taking a shower. That way the cut hair that remains on your back will be washed off.
And finally, be ready for the challenge of doing the side of your back that is the same as the hand you favor - right-handed guys will have a much easier time getting to the left side of their backs, and vice versa. But with patience and practice, I believe you can do both sides well.
It is best to check out your back in the mirror every few weeks or so to keep the forest down. One of the reasons people get frustrated with this product is that the first time they use it is with long-ignored hair that is, well, long. Regular use will not only give you practice, but you'll be trimming smaller shrubs.
This is a handy gadget that will, over time, become part of your routine. It is not perfect, but it is lightweight, cuts well when flush against the surface of your skin and more manageable than an electric shaver (or razor - gasp!) taped to a broom handle.