I bought an expansion adapter for my Zaurus SL-6000L and am quite happy with it, for my purposes anyways as a laptop-replacement.
It attaches to the back of the 6000, more or less doubling the bulk/weight of the Zaurus, and provides/includes a second full battery, a second serial port, and a second CF slot. The microphone, speaker, and reset button are still fully accessible and functional. I'm not sure if both serial ports are simultaneously usable, as they both seem to be addressed as /dev/ttyS0.
My favorite benefit of the expansion adapter is extended run-time from the additional battery. A close second is the flexibility provided by the second CF slot - I use a 128 MB SD card as additional system flash for installing programs to, the first CF slot for data storage on a 256 MB card, and the second CF slot for a 1GB card for oggv music files.
I've not done quantitative tests, but I sure *seem* to get much longer run-time with the second battery. I suspect that the battery monitor toolbar applet may mis-report status - it will say that the secondary battery is almost depleted, and appear to *not* switch over to the primary battery, shutting down instead. But it seems that what may really be happening is that the Z has already depleted *both* batteries when it says that the secondary is gone. But like I said, this is just my impression without any timed tests to confirm or deny.
cheers and God bless,
ybiC