Currently, I'm considering the SmartMedia reader module... The idea of infinite storage space for apps and documents is very appealing, and it would give me something to do with the 8MB smartmedia card that came with my old digital camera.
But I also find myself drawn to the "gizmo" springboard modules, like a GPS unit (when would I use it?), the Visor Phone module, and the mega-reference modules, like the Springboard dictionaries and medical reference books. The MP3 player modules I've seen are also nifty, but I'm waiting for an MP3 springboard module that includes expandable memory...
I think the Compact Flash reader module will accept the new IBM Microdrives, but I don't know if there's 1GB of Palm software out there to store on it... LOL.
They also have the auto-backup modules, which would be great for when I travel without my laptop... but they seem spendy when compared to a SmartMedia reader that would allow me to copy files to it anyway. Of course, I loose the point-and-click restore function of the backup module that way.
But I figure I've had my Handspring for almost a year now, and the Springboard capabilities were one of the reasons I went with a Visor... so I feel obligated to buy some. ;)