I've used all three types of the most recent prototypes -- the Dev Kit 1, the Rift HD prototype and the Crystal Cove prototype. I have a significant amount of time using the Dev Kit 1. I got mine in May of last year. I have about 2 hours of seat time with the Rift HD prototype and about 15 minutes with the Crystal Cove prototype. There has been an evolutionary improvement in each of them so far. Going from the Dev Kit 1 to the Rift HD was a notable improvement on addressing the screen door effect. Crystal Cove goes further in addressing most of the weaknesses that existed in previous prototypes. The OLED screen is crisp and high contrast. When you quickly move your head from side to side you no longer have a bad experience with slow pixel switching motion blur. You can actually peek around with the positional head tracking now. A lot of what made people experience motion sickness is addressed by the fact that the motion blur and lack of positional tracking is now mitigated. The tech that is in the Crystal Cove prototype is what I would consider the baseline of what I'd like to see in the consumer version. The only thing I would like to see improved is to eventually see the Rift use a 3840x2160 OLED. I foresee that occurring eventually. Either way, Crystal Cove is just the latest in a series of design evolution towards the consumer version. If I had to guess I'd expect to see several more experimental designs happen before we see it being sold to consumers. Crystal Cove just happened to be the prototype they had available at the time of CES 2014.
Here is a picture of one of the Rift HD prototypes I used previously. It's a bit different than the Crystal Cove prototype.