I just got finished testing out my gamecube, wii and SNES with the OSSC (more consoles to try out tomorrow). All I can say is I'm very impressed with the 5x line quality. My budget 4K Sylvannia 55" seems to not complain about any LineX multiplier so I guess I lucked out. With passthrough on 480i, 480p or 240p I get no signal but that doesn't matter to me.
240p is definitely this device's forte from what I've seen, but 480p honestly doesn't look that bad. Really satisfied with this thing.
I played Mega Man X2 up to black zero fight and experienced zero issues which makes me glad, as I've heard OSSC's worst partner is the SNES (though 5x does cut off the screen so I'm not sure what to do about that).
One weird thing though, when using Line5x on the Gameboy Player (with GBI-UL). I thought using Line5x would have filled the screen up but it didn't. Regardless though, it increases the clarity very much to make it look extremely clean. Maybe I could try my TV's native zoom function to compensate (didn't think of that).
So far, I'm extremely impressed and satisfied. I'll be testing Silent Hill, Dino Crisis and Chrono Cross tomorrow evening to see how the 240p -> 480i transition is. If it's fast, this thing will have automatically dethroned the framemeister for me.