I have used both poncho & rain suit.
I agree with Kim on one point (if your poncho doubles as your tarp, then ans. the call of nature in the wee hrs of the morning when it's raining can get you a bit wet from the rain), but i disagree with her on another point as did the prev. poster who posted as i was composing my full-length Novel reply.
An eVENT rain jacket is very nice coupled with some WPB pants. Use a large kitchen trash bag or a specially made pack liner bag (sold by GossamerGearPackLiner). You don't need a pack liner if you are using a poncho.
Primarily, however, I use a poncho for rain wear. Yes the 'spinn' fabric or silNylon (i have both) are not breathable, however, this is not a problem as the poncho does not seal tight and so is ventilated to a degree. The spinnPonchos sold on this website are very nice as are the silNylon ones sold by Integral Designs, Equinox, Campmor, and Dancing Light Gear. The ID & Equinox can be found online at BackCountryGear (if i recall correctly) & Campmor & DLG sell their own, though Campmor sells other brands as well.
If you're also going to use your poncho as a shelter, the extended length ponchos (the back snaps up for wear - or unsnaps for wear in a veritable deluge) from Equinox and Campmor are very nice. The DLG poncho-tarp is perhaps the largest extended poncho of them all & so makes for a larger shelter. Keep in mind that the larger extended ponchos in silNylon weigh ~3.0-3.5oz more than the ponchos made from spinnaker fabric sold on this website.
Just so you can try to cp. to the PacNorthWest. I live in Connecticut. Lots of rain, though almost never as much total annual precip as the wetter parts of the pacNoWest. Our wettest year on recent record (105 precip days that year i believe if i recall correctly; i think ~95 precip days is more typical), a few yrs ago, topped Washington State for precip that year - according the local weathermen (maybe it was a dry year there in the pacNoWest also??? don't know?). So, we too get our fair share of precip. T-storms with heavy rain in aft, eve, & night during late spring, summer, & early fall - 30min to 2hr duration typical. Varying light to moderate rain for up to five days straight - round the clock with virtually no let up (this actually happened in that wettest year i referred to earlier - great shades of Noah!!). Heavy rain for up to a day. Rain falls up to 4" per hour when very heavy, though 1" to 2" per hour is more common when rain is heavy. Lots of run-off & trail erosion, and low-land flooding as a result. Rain can be very noisy when it falls this fast in such large drops and is landing on your house, yard, driveway, & cars. It sure does make a racket!! A lot of overnight rain in the wee hrs of the morning for a few hrs - usually light to moderate. 'ok', i think you get the idea - not the PacNoWest, but not the arid SoWest either. Maybe, i'm a bit strange, but I love to hike in the rain (and after nightfall too).
Poncho keeps me dry even when trail leads out of the forest to a rocky hilltop. Yes, some wind driven rain may enter the poncho.
Under the poncho, I often wear a Wild Things hoodless windshirt made of Epic fabric. This windshirt coupled with either Epic trail cargo zip-off pants and various gaiters, keeps me dry in light-moderate rain for a couple of hours even without the poncho. When worn with the poncho, i haven't gotten wet yet.
If it's real hot, like during the day in the summer, I don't wear the Epic windshirt under the poncho. I just push up my base layer long sleeves, and let my forearms get wet, if I'm wearing long sleeves that day/trek - sometimes still do for some sun protection even when very hot.
My ponchos are long enough (or is it that i'm short enough?) to come just below my knees. During very hot summer days, if I'm wearing more breatheable lighter weight (not made of Epic fabric) nylon cargo pants, i'm still basically covered on the legs. The poncho extends far enough down that my knee length tall gaiters cover the rest of my legs. If i'm weaing Epic pants, i typically only wear some Montbell shortie gaiters made of Schoeller Dryskin as they only weigh 1.5oz. This combo keeps my legs dry.
I stick mainly to trails with some off-trail excursions. Getting ripped to shreds by brambles/briar patches, both clothes/gear & skin, is not my idea of a good time (unless i'm "hunting" wild raspberries & blackberries). If you're going to be bushwacking in the rain, then maybe a poncho isn't a good choice for rain gear.
GossamerGear also sells some inexpensive Tyvek-like mat'l (maybe it is Tyvek???) rain gear similar to DryDucks i believe. I've purchased it, but haven't used it yet. Seems decent enough if one is careful. Nicely made. Here's a link to it. It won't last as long as a more expensive rainsuit, but is very inexpensive so you can try out a rain suit or use it under conditions when ANY rain gear, not made from plate mail/armor might get torn.
hope this info helps.