I would characterize this as a very good work chair and an OK creeper.
As a chair, this works very well because it provides good elevation, nice cushioning , and the area underneath your seat is divided into two open 'shelves' where you can put your tools, trays, etc. I keep things I need immediately in the front half; the back half stores things I've taken off the vehicle and won't need.
As a creeper, I find that at 5'11" with a 32" inseam, I do fit on the creeper, barely but comfortably, with head supported and my butt on the bottom. If you are any taller, however, you would start to hang off the creeper. There's really not much to say in this specific mode otherwise- there's not much of an option to elevate your head, but it is quite low profile. An interesting use case with this kind of cushioned creeper rather than the hard plastic ones is that you can comfortably work on your stomach and even on your side as well.
Setup is easy, as this is a 3 piece design that requires only 3 bolts to put together (plus casters). Once together, you only need to pull a small pin to convert quickly from seat to creeper.
The main criticism I can give is for the casters. Unlike some other users, mine have not fallen apart (yet), but they feel incredibly cheap with plastic rather than rubbery wheels and don't roll all that smoothly if you have some level of texture on the concrete surface you're working on. It's easy enough to pick up better ones from any supply store, but I would rather they have a version with upgraded casters, I'd gladly pay more for that.
Overall though, this is an idea that does work in execution, and has made life for casual car maintenance and repairs much easier.
Shelly
Very disappointed! Looked nothing like the ad, I thought they sent me the wrong shirt. 
Ashley Romans