I have just been through an exercise of looking to replace my old PSU (Portable Storage Unit), my old one was an X'S Drive II which I had for three years, and unfortunately it was the victim of a work related accident.
A PSU is basically a hard disk with a battery and a card reader, you insert the card, press a button and it copies the contents to the hard disk. The differing features are card types supported, preview screens, battery life, performance, and other functionality (MP3 players etc).
I had several issues with the X’S Drive, the first was speed, it took 30 minutes to download 1GB, the second was battery life, the third was no verify function and finally there was no progress display.
Since then, there has been a huge number of these devices introduced to the market. The cream of the crop appears to be the Epson P2000, it seems to do everything and has a huge preview screen, it is also out of a lot of peoples (min included) budget.
In the end I short listed to the PD70X, and X’S Drive VP3320. I had some issues trying to get the PD70X, and there was some unknowns about the unit. In the end I opted for the VP3320. It claims to be able to download a 1GB card in 6 minutes, has better battery life, a verify function and displays progress.
I ordered this unit and a spare battery yesterday, I expect to see it in the next week or so…