Flavor's wonderswan cart is really nice, and I enjoy it. It has a micro-sd slot, but it's not used yet, and I do hope it will be used eventually. But who knows if it will ever be? At the moment I still need to rely on a software on my computer to transfer a limited amount of roms to the cart.
As for the CD systems, I only mentioned the Sega-CD because most other systems have working solutions:
- Dreamcast has the GDEmu and a couple more
- Saturn has the Rhea/Phoebe and another one is in the work already
- Playstation has PS-IO (compatibility is not as good as the other ODEs, but getting there)
- 3DO has a good solution from mnemo (forgot the name)
- PC-Engine CD-Rom is THE one missing a good solution as of now, but Deunan (GDEmu/Rhea's creator) is apparently not too far from having a working device
- PS2 and more recent consoles all have hard drive or SD-Card solutions (GC, Xbox, Wii, 360, PS3)
So, out of the remaining ones, I (personally) find that only the Sega-CD has enough good titles to warrant such R&D work.
Well, it would be nice if a solution existed for the PC-FX or the Jaguar CD, but you'd have a hard time selling enough of these to cover the development costs, I reckon.