You can get wet from sweat or wet from rain, or a combination. The waterproof breathables depend on the outside surface being not wet. So the outside layer has to shed water. This generally works well in light to moderate rains -- up to about 1 mm/hour, but fails in major downpours. You will get wet where pack straps bear down on your shoulders, and at least damp between your pack and your body. This is one reason I prefer an external frame pack. Less of it presses against you.
The poncho comes in a variety of shapes. One version has a longer back so it can be worn over your pack. I have generally stayed drier in a poncho than in a raincoat, as there is LOTS of ventilation. Some have snaps on the edge you can use to make short sleeves for your arms, or reduce the wind catching on them. Recommended.
A poncho is a pain in the wind, and it tends to catch in brush, or crossing deadfall. The wind issue can be relieved somewhat by making a belt out of elastic waist band material (1" wide plus snap click buckles.) This usually makes the brush problem worse.
Ponchos are ok for walking or standing around, but seem to constantly be in the way if you are working, Try setting 2 foot survey sticks, or doing litter pickup wearing a poncho. Ugh.
I've used ponchos canoeing too. The waist belt is necessary here, as the wind can flip either end over your vision in a hurry. I was uneasy about dumping wearing a poncho, enough so that I removed it and tucked it away when we came to rapids. Wearing your life jacket over your poncho helps a lot. Ponchos for canoeing have a longer front so your lap and thighs are covered.
This is a jacket with a water impermeable layer. The most common is urethane coated nylon. I expect now there are silicone nylon ones. They are perfect at keeping the rain out. And at keeping the sweat in. They are inexpensive, and if you are in a climate where it rains a lot or you are in a situation with a lot of water in the air (sailing...) they are a good solution. You will get wet from sweat condensation, but you will be warm and damp.
Gore tex is in the lead here, but there are several competitors. The waterproof layer in a goretex jacket is a separate layer. Good jackets have a liner layer too to protect the magic (It's a sheet of teflon that's been stretched.) This makes the jacket somewhat heavier, and a 3 layer construction is more expensive.
They aren't fully waterproof due to needle holes to sew. Good ones are seam sealed at critical points -- shoulder seams.
There are various breathable membranes that can be stuck to the fabric. These are much less expensive since the garment is then only single layer construction, but it tends not to work as well as the gore tex ones.
Wet weather clothing in general:
In general the synthetic fabrics work better. They can get wet, but the water isn't absorbed by the fiber itself. Wring it out well, and it's generally just damp, but will now trap enough air that it's reasonable warm, even though you feel like you are making love to a clam.
You have 3 different issues: Insulation, water resistance, wind resistance. Often blocking the wind over a light layer is sufficient. You will use various combinations depending on circumstances.
Fleece is warm, but has very little wind resistance. A wind parka has excellent wind resistance, but little insulation value.
Sometimes I've come up with interesting combinations:
Tracking and lining canoeing: Sunny day: Water boots, running shorts, life jacket and toque (aka watchman's cap, stocking cap, mitten hat) when I start getting chilly. Cloudy day. Add polypro top and bottom.
The poly gets wet, but it's barely damp 20 minutes after you are out of the water.
2 inches of snow the night before. All the brush with leaves full of slush the next morning -- but bright sun and warm. Shorts, no shirt. Got a blast of ice water every couple hundred feet when had to pass between two bushes, but overall was comfortable.
-15 C, bright sun. snowshoeing. Wind pants over poly bottom. Wind parka over skin top. Just blocking the wind was enough.
-40 C, 30-40 kph winds. Winter parka, two fleeces, toque and balaclava, polypro top and bottom, fleece pants, wind pants, felt lined boots 3 sizes to large with 3 pairs wool socks, leather mitts with fleece liners, and thin polypro gloves. (the latter give full dexterity, but your skin doesn't freeze to dog clips.)
At present my preferred system for hiking:
Note: I'm in central Alberta. If it rains here, the temperature drops to about 10 C (50 F) in a hurry. In the mountains you can get snow any month of the year, and even in August hard frosts on a clear night are common.
Tight weave nylon wind pants with full leg zip. This allows you to unzip the cuffs a bit so water drains faster after fording a stream, and allows you to unzip the sides at the top for more ventilation. It also means you can put them on and off without taking off your boots. Do not get the super light weight ones made from rip stop nylon unless you are on groomed trails all the time. Look for material equivalent to what is often used in kids book bags at the light end, to lightweight jean fabric at the heavier end.
A goretex parka. Avoid the lightweight ones. MEC has a line made from heavy nylon canvas as the outer layer. This is certainly proof against spruce twigs and wild rose. Haven't tested it against Devil's club.
Under the parka a minimum of a dry wick t-shirt. More commonly, a light weight polypro top, long sleeve. If it's colder I have a medium weight fleece. Usually my legs are warm enough due to the effort of hiking, but if not, I also have along polypro bottoms. These come out if the temperatures are close to freezing, and I'm walking through rain soaked brush.
If the weather is consistently below freezing, I will wear a wind parka that is not water proof. Most of the breathables don't work well below freezing if they are decent at above freezing temps. The pores in the magic layer plug with ice.
On my head I usually have a toque. I like the cheap acrylic ones that you can get at the dollar store. I usually carry two, one for day time use, and one to sleep in.
All jackets are below-the-butt length. This puts them mid thigh in front. I don't like elastic waists: They impede airflow, and I get wetter. Having a draw string at the waist level can be useful on windy days, but I use it rarely. A loose lower end means that as you move more, the garment flaps more allowing warm humid air to move through the insulation area. This, along with hood on or off, top zip, gives good ventilation.
When hiking in light rain, I often only do up a couple of snaps on the front. This gives me reasonable rain coverage, but leaves good ventilation.
On my feet I usually wear a thin pair of poly pro socks and a pair of thick wool/nylon socks. If there isn't snow on the ground, I wear low cut trail shoes.