Hookah pen is really just an e-cigarette that heats of glycerin with a tiny amount of nicotine and flavoring. When inhaled it creates a non-carcinogenic smoke. Its relatively safe, but remember you are usually ingesting a bit of Nicotine. And they are used for decreasing the amount of real hookah smokers!!!
There are just another type of e-cigg. They have pro's and cons. They may keep you from ingesting smoke and other cancer causing agents but the jury is still out on the damage they vapors from these e-ciggs may or may not cause. I looked up some of the stuff they use to make the oils and flavors for e-ciggs and saw a lot of words I was not able to pronounce. And my vocabulary may not be on a genius level but when the word has 3 consonants at the start end ends with what seem scientific descriptions of ingredients... I get a bit nervous.