I've spent a lot of nights sleeping in hammocks in a lot of places. For me, they're the most comfortable way to sleep. If comfort and quality sleep is your biggest priority, hammocks are hard to beat. With that said, setup is extremely important. Many guys who dislike hammocks haven't set them up correctly. hammockforums.net and shugemery on youtube are great places to start learning.
Another advantage to hammocks is the fact you can easily make most of the equipment yourself for very cheap. I have a 12' hammock with 14' tarp, whoopie slings, and guylines that weigh less than 2lbs, and it cost me around $150.
For cold temps you'll need a lot of insulation to stay warm, and that's where things get a little less advantageous compared to ground sleeping as far as weight goes. I've used both pads and under quilts for insulation and much prefer under quilts for comfort, but pads are acceptable (and a lot cheaper.) Generally speaking, hammocks are not as good for harsh weather, but you can get systems that will withstand high winds and lots of snow. You just have to pay for it. In my experience, when its cold, its cold, and it there isn't much difference if I'm in a tent or a properly equipped hammock setup.
One of my favorite aspects of hammock sleeping, isn't actually the quality of sleep. I love having a huge tarp and being able to do an open style pitch with a fire in next to me and an amazing view of the mountains around me. I'll close it up if I need to, but I mostly sleep with it open.
One thing is certain; hammock-ing is A LOT of trial and error. But its well worth the effort.