You can see the size and material difference between your average atomizer and the splinter+stem right? The splinter looks nearly twice as thick as most atomizers I've seen and is at least twice as long w/ the stem.
The average box-mod fits easily in the pocket, the splinter looks too big to comfortably pocket even without the stem, then you also have to transport and load the stem. I couldn't see myself using it anywhere but at home. Certainly not out in public.
I've held the prototype. I took it to an e-cig store when I went to buy a box mod. It did not look particularly different from some of the configurations they had. The stem does make it taller but in my opinion, still not terribly unusual. I agree that it's not particularly pocket-friendly. That's mostly due to the stem, which has other issues as well. To carry it in a pocket, you need a stem that you can cap somehow and there's still the issue of assembly before use. All of these issues can be remedied but the Splinter will never be a stealth device along the lines of, say, the MFLB.
The e-cig store guy was enthusiastic about it. He said that he gets inquiries about such a device all the time. He didn't seem to think the form factor or size mattered much.