I like K2 more than other "legal highs" I've tried (as Yerg noted above).
It lasts around 3 hours for me, whereas Spice lasts more like 5 hours.
It gives me a good high, really similar to the real thing. More weed-like than Spice, I would say.
It feels cleaner, and healthier, to me (intuitively, based on only my own feelings).
I vaporize one or two hits of it and I am off.
Usually I use it 3 times a week, but today I used it 4 times (I was high most of the day), and without any noticeable bad effects.
Based on what I have read, I would guess that it is JWH-073.
I use it because of the crazy drug laws where I live, and the fact that I can't even get weed.
I can get it shipped to me without any problems, whereas shipping real JWH-073 would be very risky, and I could easily end up in jail.
Since I am vaporizing it, not smoking it, it is probably the same thing, for my purposes.