I know for a fact that at least 7 of the top AI people that I listed above think intelligence explosion / superintelligence is plausible and that it's not at all clear that will go well for humans. I'm pretty sure all of them think AGI will be feasible one day and will be fairly hard to make reliably safe, at the very least to the degree that current safety-critical systems require a bunch of extra work to make safe (beyond what you'd do if your AI system has no safety implications, e.g. detecting cats on YouTube).
I can't say which people believe which things, because most of them communicated with me in confidence, except for Stuart Russell and Shane Legg because they've made public comments about it about their worries about intelligence explosion:
http://futureoflife.org/PDF/stuart_russell.pdf
http://lesswrong.com/lw/691/qa_with_shane_legg_on_risks_from...