Superbit uses the maximum data rate allowed by the DVD-Video spec, which usually produces a slightly better picture quality than the regular bitrate. The effect is generally very subtle, and almost completely unnoticable on screens smaller than 32".
The extra picture quality is most noticable in fields of subtle color changes, such as a shot of a blue sky.
It also depends on how the original disc was encoded - the MPEG-2 encoding process has improved greatly since 1998, and the extra quality you see with Superbit may just be a better overall encode.
The best side-by-side comparison i've seen is with the "Fifth Element" original DVD release vs. the Superbit release. I definitely see more artifacting on the original relase, which was compressed pretty heavily to fit all the extras. However, it is a subtle thing, and only visible on my 36" WEGA - not on my 19' TV in the bedroom.