As the price of MicroSD cards is still much lower than the Sony Pro Duo, I decided to buy one of these for my PSP.
The adapter is exactly as shown in the image on the product page here, with a slot on each long edge to accommodate the MicroSD memory cards. I formatted a 16Gb and 8Gb SD card as FAT32 within Windows and placed them in this adapter, but the PSP refused to read any data from them. So, leaving the cards in the adapter, I reformatted them using the PSP itself and it worked first time. I ended up with what looked like a single 24Gb memory card as far as the PSP was concerned. After adding some homebrew games, emulators, music and a couple of movies, there's still loads of room left spare.
To keep this balanced, my two criticisms are minor, but worth knowing: firstly, the adapter feels quite fragile because the plastic has to be very thin to allow room for the cards and so needs to be handled reasonably carefully. Secondly, it can be hard to get the cards out of the adapter because of the very tight fit and I had to resort to using a pair of pliers (gently!) to help remove a card from one side. It may be my adapter or SD card at fault, but as the adapter isn't spring loaded, you have to rely on a fingernail to remove cards using the tiny ridge on them. Not easy in my case.
For the price of the adapter and two MicroSD cards, I've still saved a fair bit of money on the price of a similar Sony (or even unbranded) Pro Duo card. My quibbles are minor and I haven't once regretted my purchase choice.
Highly recommended.