SC:DA is a solid stealth/action title on the GC, despite dated graphics and lack of online play.
Let's get that reality of the GC out of the way while we're ahead.
Splinter Cell Double Agent does the GC justice by delivering an adult oriented title of pretty good quality. In America, the SC series has pretty much eclipsed Metal Gear Solid as a recognized and applauded stealth franchise. It has fared quite well on PS2, XBox and PC. On the GameCube, however, its lack of online multiplayer has held it back from greatness. This is Nintendos' blame. The lower graphical quality is Ubisofts' blame; GameCube has more horse-power than PS2; there is simply NO EXCUSE.
Double Agent has, hands-down, the best story and most variety in the series. From snowy caves to prison riots to military bases: the mission sites and objectives offer alot. The campaign mode alone is worth the price of admission. The two player co-op adds to the story by filling in the gaps, while letting you and a buddy tackle slightly alternate objectives. This keeps the replay factor up.
If it weren't for the lack of online play----it would have gotten an 8.5!