I have a Gemini. It's a nice device -- as delivered it's a cross between a Psion Series 5 and an Android phone. In this guise it works pretty well for most things, if you like Android, though the software could do with rather more regular updates.
However, it's not really what I wanted it to be. I didn't want to run Android, I wanted a full Linux workstation in my pocket. As I have limited pocket space I wanted that Linux device to double as my phone (you know, to make calls and stuff). The hardware could do that, but Planet haven't done as much as I'd hoped on the OS side. One can run Linux on it, but it's not capable of doing all the important things a "normal" smartphone does, so I have to carry two devices or leave the Gemini at home (which is what happens).
A shame, because it's a nice toy. The Cosmo looks nice, too, but I do wish planet had spent their limited time and money improving the software on the Gemini rather than improving the hardware (which was already good).