Using in a crowd is tough with any vape in my opinion, since they see you sucking and blowing clouds, but the Indica does fit fully hidden in my palm, so it just look like I'm yawning or something when taking a drag (still I haven't really tried it in the middle of a crowd completely surrounded by the people).
I want to give a little context. It was a crowded area of people walking in a city--I was also walking. This was much easier to be discreet about, as the mouthpiece is the only real visible thing in my hand. My friend didn't even know I used it (we were walking to get coffee).
Most people aren't going to know what it is I'm exhaling because I don't/didn't exhale big clouds, and I only took one or two small hits to knock out the anxiety I was getting. And at the time, teens headed our direction overpowered any hint of the PAX I was using with their obvious recent toke (she was convinced it came from a different direction and it was really strong. I've vaped with the PAX with her in a closed room, so... I'm taking her word for it. Not scientific, but eh). That's California for you.
Back to discretion--My PAX is barely visible in my hand, and it's black. The LED is not noticeable in sunlight. It looks like a portable battery charger or some other tech, which is barely thought about in the Bay Area. The Indica is shiny, and looks like a zippo. It's beautiful and it's premium functions and portability are several reasons why I'm interested in it, but I don't think I could draw off of my Indica in a crowd without some funny looks But, I could probably stand in a smoker's section with nary a glance if no one was explicitly watching me.