Mmm… Don’t know about hardcore – I only got it so my wife & I could grab some Mario Kart action whenever it comes out and our baby daughter lets us, as we’re big (big) fans.
I find that it answers perfectly the problem of my now-highly-segmented-gaming-time. Much better than longer, ponderous (sedentary, really) games on TV-type consoles (XB, PS, GC).
Visuals have never been an issue with me, but nowadays less so than ever given how little time I can devote to the medium, so losing the magnificence of Halo 2 to the coarseness of SM64 DS is no deal whatsoever so long as there’s good gameplay…
Still only got the one game for it (SM64DS) since Xmas, still below 30 stars, only now I’ve got both the Jap and US versions – and playing the US version only (so not so hardcore :D ;) ).