I bought this cable more out of curiosity than anything else, and didn't really know what to expect as there seem to be lots of contradicting reports of what RGB scart does for you. Here's my attempt to describe its effects having tried out most of my games on it-
Zelda: twilight princess - This game has lots of very small or detailed text, and the cable improved the sharpness of the details.
Mario 64 - surprisingly, this game was enormously improved from the composite cables, making text more readable and graphics more vibrant and well-defined, making the whole game seem fresher.
Sonic 2 - the game feels more vital somehow- presumably because the lines in the background are more defined so you get a greater sense of it whizzing by.
Zelda: A link to the past - The text across the top of the screen (for arrows, bombs, rupees etc) is more readable.
Super Mario Sunshine - Oddly enough considering mario 64, i saw no discernible difference at all with this game.
Metroid Prime 2: Echoes - Detailed textures were sharper, and given that this game makes extensive use of pretty textures it makes the game look better. Note: this game runs in 480i only, and 480i seemed to look a lot nicer on my TV through the rgb cable than through composite, though i suspect that depends more on the TV.
Overall, the edges of text and buttons, boxes etc. are better defined, and the edges of 3D objects are better defined. Fine details in textures which would be lost in the blurring of a composite signal are preserved by the rgb cable.
If you can find one of these cables at a relatively reasonable price (I found one priced at £16.99) then if you're interested in improved graphics it's probably worth it, as it does represent the optimum experience on a standard-definition TV.