First of all, if you have component cable inputs on your TV, then get the wii component cable instead for an even better picture. But for those of us who still haven't got a High-def TV, these cables will give the best possible picture.
I played Mario Galaxy, Resident Evil 4, Zack & Wiki, and a few others. The difference is noticeable. Even on the wii menu you'll see deeper reds, greens and blues. Small text will look crisp and easy to read. Personally, I think it should have been boxed with the wii. So why have I given it 3 stars?
First, they lost a star because the RGB cable doesn't carry the WSS (widescreen signal). For me, I only have to press a button twice on my TV remote to change it from 4:3 to 16:9, but other TV's aren't so easy to work with. If you can't switch from 4:3 to 16:9 fairly easily, just don't get the cable.
Second, they lost a star for the price. These cables should have been bundled with the wii, but they weren't. The fact that you have to shell out £18 for them is near-robbery.
It's nice that the RGB scart carries the auto-on signal (when you power up the wii, the TV will switch to the right channel by itself), so how they could have included Auto-on but missed WSS is beyond me. If there's a 3rd party RGB cable that has the WSS signal, get that instead. It'll probably be cheaper, too.