Although S-Video is good, it can't match the Plasma VGA or Component routes.
RGB to Plasma VGA is still the ultimate interface, and keeps your component and video inputs free for future use. The only mode that you don't get is the "Just" zoom mode, the one where it pulls the side out to fill the screen. Personally, I've always found it a bit weird, especially when watching motorsport.
RGB to Component (YUV) will give you a slightly (very slightly) softer picture than the Plasma VGA. It does, however, allow for the "Just" zoom mode. Therefore, in terms of display options the closest (if not identical) to the S-Video input.
Which do I use? Both of them. Plasma VGA for my DVD, and Component for Sky.
You will also loose control over colour, but the colour is very accurate via VGA, well I always found it was. Main difference is a much cleaner picture via RGB2VGA, less video noise. I suppose technically there will be more colour bandwidth, but whether or not you will see this with a crap source like Sky, I don't know. Every little helps I suppose