I own a couple of Hennessy hammocks and like them - but am not sure I'd use them as a survival shelter. One issue is lack of flexibility - without suitably-sized trees, they're challenging to set up. Most notably, a bottom-entry hammock doesn't make a good ground shelter (I've done it with the Hennessy and some hiking poles - definitely not ideal).

I think a lightweight tent or a bivy sack/poncho tarp combo are probably a better survival solution, unless you live in an area where you're sure to always find yourself around suitable trees and in suitable temperatures for the hammock.