I have one. I got it this spring because a lot of my electronic gear is no rechargeable via a 5v USB plug system. This seemed to fit the bill nicely. I charged it and tested on a few liters at home and then took it on my most recent trip. It worked fine for the first liter of water, but during the next use it gave me the "one-green, 3-red flash, repeating" LED error code, which is "UV lamp failure, replace lamp" problem. So the lamp which is supposed to last 8,000 hours is acting like it has issues after less than 5 minutes of total use. I tried recharging it in the field but that was not the issue. I carried the useless thing on the rest of my trip. Luckily I live in a place where finding good water is not an issue. Back home it kept acting poorly, but then randomly decided to work again once.
Mine is totally unreliable. I called the company and they offered to service it for me so that I am "totally confident" in the product. Well, I doubt I will ever be truly confident in it again...
The body of the unit will fit into a bottle mouth with a 38mm inside diameter.