I think the D-Pad's fine, actually, but the one on the GBA SP is crummy because it doesn't stick out far enough.
Isn't the D-pad for the GC the same design/size that they've been using forever? The N64, VB, SNES, and I think the SNES all had the same didn't they?
Doesn't compare to the the Sega pad IMO, but it does the job fine.
Design, yes. Size, no. The Gamecube d-pad is quite a bit smaller than the ones for previous consoles.