I've owned a Hennessy Hammock for quite a few years now, and have used it in all kinds of weather. I like it a lot - it's incredibly comfortable - but it does have its quirks.
I don't like using it for more than one night in bad (especially wet) weather. Not because it isn't dry - properly hung, it will keep you nice and dry - but simply from the lack of storage space. For instance, there's no place to put your rain gear or boots where they won't get wet in a driven rain. In cold weather, there's no place to put your overcoat.
This can be partially handled by pitching a tarp over or next to the hammock, but that sort of defeats the "traveling light" aspect of the hammock.
Of course, it's darned near impossible to dress inside of one. Those who can are a lot more flexible than I am!
As for cold weather, I've slept in mine down to about 28degF and was quite warm. I use a Big Agnes bag, coupled with a RidgeRest closed-cell foam pad and the popular fleece/foam window shade mod that you can find all over the net. In fact, if the weather gets much above 40degF I sweat too much in this combo, and I'm a notoriously cold sleeper.
Great product, but consider your needs carefully. I love mine - in fact, I spent this last weekend in it - but it's not for everyone. If you like bivy bags or the very tiny one-person tents, you might be a hammock person.
-=[ Grant ]=-