While it's not a product I recommend using, It is safe to use, as it shouldn't ever delete any valid registry keys or needed files. I've used it on a number of computers, and it didn't cause any problems. In addition, I've checked the scan results of its registry cleaner on a number of computers, and it had no issues with false positives. As mentioned, some of the sites that recommend it will make money as affiliates. It's also worth noting, that in some cases, it will make computers run better. So if a review states that it made their computers run faster, it probably did do just that.
Some system utilities and cleaning software can cause problems due to deleting needed registry keys, or in rare cases, needed files. Because of this, some people consider all such software to be harmful. But really this is not the case at all. When I use such software for the first time, I manually check what it wants to delete, to make sure that nothing needed will be deleted, rather than blindly letting it delete everything it wants to.