I ran Black Canyon recently and the poison ivy was in full bloom. The plants had berries the size of moose balls hanging off them. I avoided the tree-sized PI as much as possible but inevitably got some of the oil on me. I hiked in shorts, my dry top, life jacket, and helmet.
When I got to cave camp I left all of my gear on and rubbed Technu over my helmet, lifejacket, drytop, shorts, legs, kayak, and paddle. I waited about 5 or 10 minutes then jumped in the river and washed everything. I then took off all of my gear, dried, and did another round of Technu.
I didn't get any rashes. It's definitely worth spending a few bucks to avoid a week of scratching.
On a side note, one of my buddies had to hike out at Locomotive on Giant Gap on the North Fork of the American in California. As he climbed out of the canyon, he had to grab hold of poison oak. Well, at some point he took a piss and got the oil on his dick. He's allergic to the stuff and had to be hospitalized. His penis swelled up to the size of a pumpkin. The nurses and doctors were so amazed that they took pictures of him to include in medical journals and brought in interns to see the monstrosity.
He would have given his left nut for a bottle of Technu.