i have one. there are many reasons to own it:
eternal darkness - great game, great story, great gameplay. it shits on resi evil and silent hill in terms of scaryness
metroid prime 1 - took one of the best franchises ever, and successfully changed it from 2d to 3d. it is a classic and one of the best games ever made. it is a first person adventure game which plays alot likea first person shooter.
mario power tennis - yes it does have power shots but it isnt meant to be a tennis sim. by the way u CAN turn them off in multiplayer to play traditional tennis. and this is an awesome game which doesnt take itself too serious!
mario kart - i agree, quite shabby compared to earlier incarnations..
mario sunshine - i agree crap. they will release mario 128 next year which is like the earlier marios and will rock.
resident evil series - theyhave remade some of the resident evils with much enhanced graphics and gameplay. resident evil 4 is the most antisipated game on this system and will be launched as an exclusive next year.
geist - look out for this game next year. first person shooter with a twist and is getting alot of hype.
the system is probab;y the strongest system in terms of mulitplayer gaming, so if u have lots of people to challenge it is really good trying out mario tennis/kart/party, smash bros and many more.
btw - enjoy ur new system. good choice b/c the cube has quality over quantity in terms of software