I have one of these and they are ok to get a signal out, but suck in many ways.
VGA->Composite is much better compared to the other variant, Composite->VGA. The latter one will give you all kinds of colorful, noisy and crappy image, so stay away from those. Anything around 20€ is complete trash. SNES, C64, NES, old hardware like this sure will not be usable with those. Wii is ok however.
VGA->Composite does work, but usually requires adjustments because the picture will be very much off-center. Those are not saved when you remove the power so have fun doing it again and again. And don't expect to be able to read text at 800x600. 640x480 is barely possible to read when you know what you're looking for.
And even worse you're going to set it up to work fine at one resolution but then you go to another (eg. by dropping from windows back into DOS) and then it'll be completely messed up again because the thing only has global settings and not per-resolution.
Here's a pic i took some time ago at 640x480 with exactly the same converter as seen above, after careful adjustments:
Go figure if this quality is ok for you, or not.