The Integral Designs poncho is almost the exact same dimensions as the small stock hex tarp from HH. It is one inch shorter in overall length.
As such, I used it instead of the stock hex tarp on a couple of trips when I first started hammocking, rigged in the same flying diamond shape as the HH tarp. It kept me dry for a 6-day trip where it rained nearly the whole time. It does work.
The only problem is that, just like the stock tarp, it is so small all you can do is pull it down close to the hammock and stay inside whenever it rains. After 5 days of feeling gradually more and more claustrophobic, I swore off hammocks and didn't use one again for two years!
Now that I have a larger tarp, along with a hammock that allows me to sit up, I can enjoy the front porch & recliner effect that is such a joy after a day of hiking.
For shorter trips where keeping weight low is your goal, the poncho is okay, especially if it doesn't rain much. But when you're out for a long time, it's nice for your hammock to be your home, not just your crashpad.