Well at Christmas in the US it was out-selling the XBOX 360 and XBOX...Now it's dropped down to just below the XBOX again, but there's still a strong market in the US.
Sonic Riders has been advertised over there as basically a GC exclusive, so whilst the PS2 and XBOX versions bombed, the GC game cracked the Top 15 in last month's US chart. The same is true in Europe, where Sonic Riders GC has had stronger staying power than the other versions.
Aiming at the right market sector works...It'll be interesting, in this respect, how Super Monkey Ball Adventure does. After all, SMB and SMB2 have both done better on the GC than SMB Deluxe did on PS2/XBOX...