I only have S-video for Gamecube, but I think GBI ULL's 240p looks better than both the GB Player's 480i and standard GBI's 480i. It's still not what I would consider great due to the blur and color bleed of s-video, but I can't justify $240+ for that damn Component cable.
RetroRGB has a few comparison images up on their site. http://retrorgb.com/gameboyplayer.html
Swiss 240p looks significantly better than GBP 480p (blurry). Super Gameboy 240p is still superior to both.
I run GBI ULL 240p via RGB and it's fantastic. I know that's not a viable option for Americans, though. It's a shame.
p.s. I've been lurking this thread for about a year and my GAF account only got activated recently, will have to do a big post explaining my setup sometime soon. Used to have two PVMs but I switched to a Framemeister about 6 months ago.