roughstuff.. in response to your questions:
comfort.. i, personally, am comfortable.. there was a period of time when my knees would hurt, as they would try to bend backwards with the curve of the hammock, but i just put some clothes in there to prop them up at an angle and it fixed that entirely. i sleep on my side a lot, and when i do i place the clothes between my legs, as is ergonomically advised when sleeping on your side in a bed... and i put a clothes-pillow inside my sleeping bag for my head..
i'm no doctor, i don't know if how i sleep is bad, but if how i sleep in the hammock is bad, then probably how i sleep on the ground or in a bed is bad too. what is a "truely correct sleeping position?"
the hammock is not just good for hills, but i've slept over streams and such also. i'm pretty stable in it, although other people tend to fall out if they aren't used to it. i fell out twice. once the hammock was partly on the ground and i was on a hill, so i sort of rolled out. and the other time i was drunk, and that's just what one gets for being drunk.. i've slept in it many other times intoxicated with no problems.
if you are considering using a setup like this.. get what i call a balance bar.. i use nylon climbing ribbon and carribeaners to tie off to trees.. and i run another strip of ribbon above me to grab onto while getting in and out of the hammock, or reaching for something on the ground.. the tarp also rests on this. this extra ribbon can be used to extend your reach if all the trees are a large diameter and far apart, however i perfer to not do this if possible.
the one and only drawback i have found is that in the winter it is COLD. normally you have the ground to keep your heat below you.. you warm up a patch of ground with your body heat.. in the hammock, that heat is lost.. the sleeping bag is compressed underneath you against the hammock and it's insulating power is lessened. you can pull the tarp over you and that keeps in some heat, (especially a canvas one) but it can still be chilly if you don't dress for the cold..
i'll wear a couple pair of woolies on my feet, and a couple pair of thermals twards the beginning of winter, and then ditch the hammock for the winter and use a wool blanket (a matilda, as i go a-waltzing matilda) as a sleeping pad.
if you were in deep snow (3+ ft), you might be able to make it warm by building insulation walls, or digging down and staying in a showhole with the hammock, but i don't have any experience with this, yet...
if i'm in a place with no trees, i lay my tarp on the ground.. and if it rains, i just fold it over me. i'm in the southwest now, and even if i get completely drenched i'll be dry by 10am.. of course, i like trees, and generally stick by them. i seem to recall you being from the eastern part of the US where rain can last awhile, and humidity stay high after that, which could lead to longterm discomfort if you had no way to dry out.
also i compared weight on my 1-man MTN hardware 3 season backpacking tent and my setup.. the canvas tarp makes my setup a couple lbs heavier.. but it's so worth it to stay dry. (tents, even good ones, are almost -always- damp after a good nights' rain.) i don't have my 3-man anymore to compare with that, as that is the tent with comparable space to the hammock/tarp.
hmm.. i sure am longwinded. hope this helps.