I've got about a half-dozen hammocks...Hennessy, Speer, Warbonnet, ENO and Mosquitehammock. With a little imagination, hammocks can be used without trees (as Tony demonstrated quite effectively) and they have few disadvantages unless you don't sleep well in a hammock or the temps drop to wear additional insulation pushes weight to tent equivalents.
My first hammock was bottem entry Hennessy like kgd's. I still like that version for grab-and-go bags as they are fast to put up and stupid proof for entry and exit and little fear of ripping zippers or mosquito netting. My preference though is to have a double layer bottom...it just makes it easier to secure a sleeping pad when the temps get a little cooler on the bum.
I also prefer a separate fly. Not only can it give you more coverage (I like 10x10 or 10x12 foot size), it gives you a lot more versatility. They are easier to open up for a better breeze even if it's raining but hot and humid. It allows you to set up your over-head cover first so you can keep your kit dry and still set up (or take down your) hammock. If the temperatures are really extreme, you can still use the seperate fly for any number of improvised ground shelters and with a bigger fly, you can actually seal your hammock up from horizontal rain...keeping you very protected and dry.
There's a lot of good info on hammck forums as mentioned and looking into the various suspension methods is worth the research. Mine aren't the lightest, but they are extremely fast. A quick wrap of the tree straps with a cinch buckle gets my hammock up (and taken down) and secured in under a minute with some practice. Experiment a little and find what works best for you.
Weight is the other big advantage, especially when family camping. I've packed the 7-8 pound 4 person dome tent on a few trips...never again! The kids and wife all now have their own hammock and my pack weight as dropped several pounds. If you find you sleep well in a hammock (most do), you'll be amazed at how good a night's sleep you'll get when your out. I actually sleep better in my Blackbird than a do in the bed at home!
Thanks everyone ! I'm still looking around for a good deal on a set-up ! I don't want to put alot into it until I see how I'm gonna like it !
My recommendation is to look for a deal on the bottom entry Hennessy as it's a quality package deal and you'll get some great use out of it as you evaluate a quality back-country hammock set up. Another option is ENO (Grand Trunk and Byer are other quality, inexpensive alternatives) and a surplus mosquito net. Any tarp that covers you're hammock area will do for rain cover and you'll get an inexpensive set up to evaluate if hammocks are for you...if they are, you'll be looking at Clark and Warbonnet hammocks very soon

. I actually still like my ENO and carry it on day hikes...it packs down small and can be set up for an afternoon nap or just relaxing as you weight for your water to boil or just to relax fish or fauna-watch.
Side entry or hope top hammocks have the advantage of using the hammock as a camp chair and also make it a little easier to put your boots on in the morning.
For summers/hot temperatures or insect active ground
Ill never consider another tent (unless the area bans hammocks:grumpy

. Outside of glacier camping, hammocks can be used just about anywhere. My comfort level goes down to the low 40s before I start considering a small tent. With a decent pad and Ive actually rigged a poncho liner to serve as an under-quilt, I maintain my comfort without adding specialty insulation that increases my standard pack weight. Even if the colder temperature includes that cold under-draft, I have successfully lowered my hammock to ride on a pile of pine branches (covered with some Tyvek to protect the underside of the hammock) for insulation and used the fly to seal off the direction of wind all the way to the ground (ala lean-to set up).
Give the hammock a try and if you find you sleep well, youll never go to the ground unless held at gun point
ROCK6