I was typing this the same time Elevatn was posting. It repeats alot of what he said.
Yes, it does what it claims it will do. The first real test for me was in Tellico. Alot of rocks, alot of times one front wheel is off the ground. That's where it made the most difference to me. As far as the steering is concerned, in 4WD, it seems to steer easier than before. In 2WD, you don't know that it is there.
The DGL comes with a small bottle of additive. It's only an ounce or so. If you change your diff oil, you will have to put in more additive. I have not changed my diff oil since the install. I assume, you can order more from Highlifter. Good clean diff oil and additive is important to the life of the DGL. If you get water in your diff and do not change the oil, you will have problems.
It requires no special maintenance, other than the additive. I check all my fluids after a hard ride, and so far so good.