I know its obvious, but you should mention that using a slow micro SD card makes the game load certain parts slower (but plays the game at full speed once the data has been loaded). This is particularly noticeable in Mario Kart 7, navigating the car selection screen and starting a race takes significantly longer than on a retail cartridge if you use low class micro SD. The lack of online multiplayer removes a lot of playability of this game as well. Having to make sure that you close the application in order to save also detracts from the overall user experience.
Aside from these points, I have had no issues with my Gateway so far. I was worried that the lack of multi ROM would be annoying, but really I don't change games that often, and it's only a 2 minute wait.
EDIT: Okay I just encountered the save issue.