What I liked:
Tracks - Although there needs to be more of them, the tracks presented here are really really nice.
Double Up- I really like the ability to have two people per kart. That means I'm able to have another person carrying more powerups/goodies.
What I didn't like:
Cheating - Once you get to the last 2 cups in the 150cc class races, the cheating from the A.I. is just out of control. They'll hit you with anything they have. I can't count the amount of times I was heading down the last stretch in first place only to be bombarded by red/green/Bowsher shells, lighting bolts, etc. It gets a little ridiculous after a while.
Short - You could probably finish this Mario Kart in one sitting.
Summary:
Mario Kart Double Dash pales in comparison to the awesomeness that is Mario Kart DS, but what is here is fine. The only things I'd change is the intensity of the cheating A.I. and I'd make it alot longer.
Overall I'd give Mario Kart Double Dash (GC) a 3 out of 5.