I played it when it first came out, and I felt it was challenging, but extremely fun (much more so than the original SMB). More than anything, it was fair and rewarding - especially those insane "full speed running, jump on trampoline, get blown sky-high, land on another trampoline, then get blown sky-high again" routine (starting on level 6, I believe?).
Will get the VC version, just for posterity's - and nostalgia's - sake.