I've always used an ancient GBA flash cartridge that needed a parallel port connection to work. Since it is virtually impossible to buy a new laptop with a parallel printer port, I decided to buy a new flash cart that worked via USB. This will save me digging out my old Windows 98 laptop.
The idea is that you copy the ".gba" files onto the microSD card using whatever method you like. I plugged it into a card adapter and plugged that into the port on the side of my laptop. Hilariously Linux managed to mount the card read-only, which was quickly followed by a lot of searching for what could be the cause of the "mount: block device /dev/sdc1 is write-protected, mounting read-only" message. Turns out I had the lock switch in the "locked" position, making the card read only. Durrr..
The build quality of the cartridge is pretty bad, sadly. The shell is a couple of bits of plastic with a slot for its own micro SD card adapter. The microSD fits snugly into the adapter, a bit too snugly initially as I had to give it a fair old shove to get it in there. The adapter then fits equally as tightly into the cartridge itself. Once it's in you need to apply a little too much force to get the adapter out again. I'm worried something will give sooner rather than later.
On a phat DS: everything seems to work. I didn't realise this at first, but the cartridge boots up into DS mode, from where you can select GBA games from the menu and they boot in GBA mode.
On a GBA SP: everything works too, though my "GBA Rogue" port didn't launch :-( I tried a new build of PocketBeeb I've been semi-working on and that did boot on the GBA and DS, so I think perhaps it is a problem with really old homebrew. I also tried an English-patched ROM of Fire Emblem 6, and that worked too. Launching large games takes a while though - I guess this is because they get copied from the microSD onto internal RAM that the actual GBA reads. When I say a while, it takes maybe 10-20 seconds.
The cart boots up into Chinese on the GBA the first time you run it, which threw me a bit. The fourth option down changes the language to English though. One thing I'm not sure about is what the point of overwriting the flashcard "NOR RAM" is - I think this is similar to the old-style GBA carts, but all it seems to do is corrupt the main menu.
Saving seems to work OK too.
Ah, just tried Powder 117 on the DS - it doesn't boot there, but it did boot on the GBA SP. So there are some compatibility problems with homebrew games it seems. Unless Powder does something odd on start up, like checking the hardware type, causing a crash (I don't think it does though). I might try building it from source with a newer devkitARM, see if that fixes things.
All in all, not too bad. Better than having to use the smaller capacity, slower and obsolete parallel-port based flash cartridge that's for sure. Being able to load multiple GBA files is a bonus - the only way to do that on the old cartridge was using pogoshell (on Linux), FTP-ing the file over to the Windows 98 machine, then writing the ROM there with the special software. "Clumsy" is the word.
This new cart was a little pricey mind, costing €35 with a 2Gb card - although the older GBA cart I cost me more back in 2001 I'm pretty sure - but nowadays you don't have much choice about where to buy this stuff. I got this one from r4ds-ds.com and it took about 2 weeks to arrive from Hong Kong to here in Spain with the free shipping option.
Here is a link to the latest firmware