Yep, varenicline can be dangerous for some. On the other hand, Chantix has helped some completely quit smoking as well. While I'd NEVER use Chantix, because of my own personal preference, I won't dismiss the possible positives that Chantix can bring.
I'd never recommend it to anyone, and at the very least, if someone I knew wished to take or has been taking it, I'd just try and make sure that they were aware of all possible positive and negative repercussions of taking such a controversial medication.
Edit: While I say I'd never take Chantix, I should re-phrase, if my only two choices were either smoking or taking Chantix, I'd take Chantix, no doubt about it. Fortunately, those aren't my only choices...