I've smoked summit k2... and a few others. All of the highs I had were not enjoyable. Mostly.... I felt like I could feel my organs moving around. I didn't get any weed-like high except the other worldly kind of feeling you get subtly with weed.... like.... you are outside looking in? It's pretty intense with k2, it feels like that is the only part of the high... and then you just... feel your lungs opening and closing... you can see it happening in your head some how... and your guts moving around in your stomach.... its fucking creepy lol.
Nice guy answer: Don't smoke it, the high sucks, its VERY uncomfortable, most likely very unhealthy, and probably very addictive. I have also heard it kills your enjoyment of pot.
My true thoughts: If you are completely out of weed.... and you cant get it anywhere else... and it suits you in your current predicament for whatever reason... buying a bag of summit or whatever K2 will knock the edge off.... but use heavy moderation. Only when it is the only option... and even then... its not that great of a high.