I have one. It's fairly limited so you are forced to pay attention all of the time and think about whether you need to take over. You just aren't doing the menial tasks of keeping the car centered in the lane or keeping a safe following distance.

If the system does something you don't like you just grab the wheel or touch the gas/brakes (the control messages it sends are lower priority for the car than driver inputs). It's also designed to disable itself if it sees gas or brake input.

It's really best considered as an upgrade to the stock lane keeping assist and dynamic cruise control found on most cars. Compared to the stock systems it's much better behaved (my stock Toyota system will happily drive straight across multiple lane lines in certain lighting conditions and it astounds me that such a thing could be sold), but it is a long long way from Level 4.