Math 55 isn't nice because of how much math it covers (you could take equivalent classes that cover the same material, just split up, at the same or higher level). It's nice because of the bonds it forges between the students, who are usually the top mathematical talent at Harvard. I'd suspect that it's these bonds that help the students throughout the rest of their mathematics education rather than their incoming ability or what the class teaches.

If you look at the problem sets, they're very hard (especially given that the students are all freshmen - a few of whom haven't ever done advanced mathematics before), but not unreasonably so (not really even beyond the top math majors of most top schools, I'd wager). Having made friends in a trial by fire like the 55 a/b series though, is a key to surviving later ones.