People's testimony can be misleading for a variety of reasons. Person A says 1080 is not much different than 720. Person A doesn't realize that he has 1080i, and due to the nature of how interlaced signals are displayed, his "actual" resolution as the human eye sees it is the equivalent of 720p.
Person A says 480p is the same as 720p (or 720p vs. 1080p); Person B says, "FUCK YOU! I noticed a major difference." Neither person realizes that the distance at which you watch television, as well as the size of the television, makes a huge difference in whether you can discern the HD quality changes in each. Being further back from the TV, you'll notice less of a difference in HD between 480p, 720p, and 1080p, generally speaking. Sitting closer to the TV results in a person noticing more of the HD difference, again, depending on the size of the TV and exactly how "close" the viewer is. Most studies show that, for your average sized flat panel TV in your average sized family room, most people sit too far away from the TV to realistically be able to notice the difference between 720p and 1080p.
There are too many variables that go into the differences between the three HD signals (including the capabilities of your TV, type of TV, what device is sending what type of signal to the TV, etc.) to simply state that one is clearly better than the other. But, just to simplify things, they have made three basic categories for HD, and tout that the higher the number, the better the picture.
Generally true, yes, but a variety of things that you do or don't do will affect how "mother fucking awesome holy shit balls!" the difference is.
Like I said, go into the purchase expecting a slight change, because if you buy the cable thinking that it will definitely be a "night and day" difference, or otherwise will bring you to a sexual climax of epic proportions, then you may very well disappoint yourself.