I was really excited about QS and waited for the Basis to be released for a couple years. I snagged one in the initial preorder, so I’ve had it from the beginning (~1.5 yrs I think). AFAIK it still has the more sensors than any other wearable device out there. I agree with @technophobe on the occasional HR irregularities during activity, but it seems to have gotten better for me over the course of several firmware updates. I chalked it up to kinks with their LED through the skin method of getting HR, but I forgave it since this was one of the first devices I came across that could take continuous HR measurements without a chest strap.
For me their biggest weakness is the mobile app on iOS. I’ve heard the Android app is better and I believe that since it was initially released several months earlier and by a different dev team. Originally (and for a long time) the iOS app couldn’t even view data, only sync to their website. It’s gradually grown better with updates, but still doesn’t approach the data view available since day-1 on the Basis website. However, since I mostly use it mobile, my data viewing experience has been limited.
In the past month or so Intel completely bought Basis. This could be good for much needed dev work on interface and sensor accuracy; or it could be bad if Intel just treats this like a semi-neglected pet project then sells it again in a year or two.
Personally, I’m continuing to use the Basis, while pinning my hopes on the much rumored Apple watch. I know, I know, [insert fanboy comment]. However, Apple could easily solve my QS data viewing woes that are the Basis weak point. Also, if they utilize all of the sensor technologies that are the specializations of the employees they have been picking up in the past year, their device could be a game-changer.