When I first bought my DS Lite, the screen felt a little weird. I hadn't owned a handheld since the original GameBoy, so I was worried that my eyes were getting too old. Now I don't even think about it. I can play any game on there without any problems whatsoever.
The length of NSMB is definitely adequate, and the alternate exits/star coins add enough replay value. Plus, some of the mini games are quite fun. I will absolutely be playing this straight through a second time in a few months (I still need to get a few star coins, of course).
It's so difficult to compare this to earlier Mario titles, because they've been classics for such a long time. Time will tell if this is a true classic or not, but I absolutely have ZERO regrets about paying full price (and I almost *never* pay full price for a new game).