The current G6 loaders for NDS mode and GBA mode are pretty good. I've never used any other flash cart to compare, although I have used plenty of Xbox 'dashboards' which are similar in nature.
However, they are not perfect. Regarding the NDS loader:
1, If you scroll down with the D-pad to the second or third 'page', then press up, it goes up a full screen, not just one line.
2, No autorepeat if you hold up or down on the D-pad, making it slow to choose the rom you want if you have a lot of them listed.
3, General slow screen update when drawing/scrolling the menu (the DS has got more power than this!)
4, No auto-sorting of the roms on the loader itself (although you can use FATsort on a PC to do this).
With the GBA loader, I dislike the fact that you have to control a mouse-style pointer on the screen to make selections. I'd rather than D-pad moved from one rom to the next instantly, stepwise.
Plus there are improvements that could be made to loading, as I've mentioned in my
Improving the save system thread.
Still, I don't want to turn this into a moan-fest at the G6 team, rather have this as something positive for third-parties to work towards.