Yep, practise helps you KNOW your gear.

I too was out walking and trying out some gear. Last Saturday, I was out in a steady drizzle with my new sil-nylon poncho tarp that I wanted to try out. Note: I normally use a goretex shell and rainpants as needed for camping/hiking. This was in a no camping wildlife area nearby so I was just out for a short walk. I purposely did not take along a hat. I wore simple lightweight pants, socks and trail shoes, medium weight merino wool long sleeve shirt and cotton vest underneath the poncho along with a lightweight buff around the neck. No pack. Temps were about 11-12 degrees Celsius with 20k winds.

After a minute of figuring out the snaps and where to put my arms I strode off for the 2k walk. When I got back to the vehicle I evaluated.
1. A hat, any hat but a wide brimmed hat especially would have been a welcome addition. I often use my Tilley hat with my rain shell because I don't like closing the hood. I prefer the visibility and ventilation of an open hood. As such without a hat, the poncho hood managed to stay in place most of the time but occasionally slid forward and covered too much of my eyes.
2. I wondered what would work best if I were to use this as a shelter. How would I seal the hood from dripping rain onto me? Simply cinching the hood would not suffice - perhaps I should attach/pack a ranger band along with the poncho?!
3. My lower arms, wrists and hands got wet. That means my long sleeves were wet and would take a while to dry out. Perhaps I should have pulled my arms inside the poncho, eh?
4. My poncho/tarp has an elastic cinch along the bottom rear edge. The elastic snagged on a branch and pulled it partially out.
5. Aside from my lower arms, wrists and hands the rest of my was quite dry. I was able to ventilate and not have sweat build up like I would in heavy rain gear. In a sense I was even more comfortable and flexible than using a goretex shell, however even with goretex a long rain would eventually soak through. The wind did not seem to drive any moisture through the "sleeves" of the poncho and my legs were dryer than if I had wore a jacket. I could easily have wrapped some cordage around my waist it the poncho was billowing too much but didn't have to.
6. I was able to shake off the moisture and hang the poncho/tarp up to dry when I got home so I didn't evaluate its drying ability in the outdoors.
7. It is a compact and lightweight alternative to a goretex shell (about 1/2 size & weight) and I will likely pack it in my day pack. And obviously I will take it to my next 'Riders' football game! The green poncho blends perfectly with the team colours.
8. I do however question the rationale of the dual usage of a poncho/tarp for some conditions. If the rain is start/stop and gives you time to set up that might be fine. If it is raining continuously and if this my only shelter how can I set this up as a tarp and stay dry at the same time? By bringing another shelter? (Bivy bag, 2nd tarp or even tent?) Not very practical as a one item doing double duty, but then again, on the dry prairies I may not have as much need to use it as a poncho.
9. I did not attach cords for use as a tarp shelter but if I did, I would probably use something like triptease for visibility rather than black tarred bankline. But that brings me to my next point - should I keep the cords attached and remove as necessary for a poncho or keep them on to use for the shelter? I can keep the cords stashed in the same silnylon ditty bag but not too much length. And I would need to keep stakes separately since there is no way the bag has enough room for them.

Like unimogbert, I too share the same thoughts about knowing your gear, understanding its limitations. Mostly by doing exactly what we did - using the gear in known comfortable conditions first, then expanding out and testing the gear in lesser conditions before finally settling in determining how it matches your skill set. But unlike unimogbert, I don't have kids that I can do a lessons learned post trip examination.