I got the smaller of the two (550 ml), holds a tad more than 18 oz. I also have the larger 21? oz. Vargo version that I got here on Massdrop last year. The designs are very similar; the only difference is the loop attachment on the cap, and the Keith one came with a slick neoprene sleeve, which works well with hot and cold liquids.
This bottle has been my only daily water vessel for the last two weeks: hot coffee (sometimes iced) in the morning, iced coffee at work (we have an ice machine!), iced tea, plain water with Mio or other drink mix, decanted (lol) soda, and sometimes just plain water. The only thing I didn't try was heating the bottle on a tiny alcohol stove (which was another reason I purchased it), but so far I love this thing. It is not a well known fact, but titanium makes everything taste better, even sour milk--just kidding! But there is something about titanium never interacting with the contents even (to me) better than stainless steel. I could rinse out the leftover coffee, put in iced tea, and there was never a taint from the coffee. It "rinses clean" or "isn't absorbed" or some other semi-scientific sounding characteristic. (To be fair, the Vargo one has the same characteristics).
At 7 3/4" tall, it's the perfect size for a backpack water bottle sleeve. I never felt like it would fall out, but this depends on the backpack of course. It's perfect for the conference room (nowadays you can show up with anything: "must hydrate!"), the park, or just carrying around. It's slightly too big to comfortably fit in a pants pocket, but doable if you have no other option while you juggle your cell phone and a child's hand.
The neoprene sleeve serves five purposes: insulates the contents from your hand, retains the liquid's temperature longer, absorbs condensation from cold liquids, provides a better grip via the surface and thru two loops on one side, and prevents the bottom from being scratched or scratching other surfaces.
Negatives: I added a paracord tether to the cap, not only so I wouldn't lose it, but so I could leave the bottle open without worrying about contaminating the cap (it hangs down on the side, but doesn't touch the surface). I remove the silicone O-ring to clean underneath as that can get "gummy," but I wonder where I could ever get another O-ring.
I really like this bottle. Let me put it this way: if I lost it, I would buy another immediately.