The Multiple Man as Peter David is writing him (and the title he stars in) and Bucky Barnes are two of the best written characters in the DC/Marvel "mainstream". I think a lot of people can identify with his personal conflict. With himself(s).
Multiplication powers (along with size-changing);The underrated super-power of comics. Also, a generally all-round great character these days.
Madrox has long been one of my favorite characters. Since he can assimilate the knowledge of his duplicates he's pretty much the most versatile character in mainstream comics. Still, X3 pretty much thought of him as "guy who wears cool shirt." That's only 75% of his appeal!
Multiple Man brought me into comics. I fell in love with comics reading the X-Factor (lead by Havok)terrible art and all.Multiple Man keeps bringing me back into comics. I'm glad Marvel considers him an important character.
But really, how can you NOT love Madrox?