There are pros and cons to this technique. If you are concerned about matching corners, using this interfacing technique really helps. I think it would also be very helpful if laying out a completed design such as Angela's Mario blocks. Although, with several extra steps, I'm not sure it actually saves time. I made three blocks using the interfacing technique. I made two blocks piecing normally. Because I am sewing my squares together randomly, I found that sewing my blocks the old fashion way was a bit easier. The trick is consistency. I have a 1/4" foot with a guide that makes a huge difference for me. I just chain pieced, and the block came together relatively quickly. I pressed seams to alternating sides for each row so the seams nested well when matching corners. Overall, it really wasn't that painful. It's pretty brainless sewing that I can do while watching a show on my laptop.
In related news, my first official Pellon tutorial is up! As I mentioned above, I made a pillow using one of my confetti blocks. The full tutorial can be downloaded here. In the tutorial I detail the process of using the Stick-N-Washaway. I found that the blocks can shrink up quite wonky as they are drying. I decided to clip my blocks to an acrylic ruler while they dry to minimize crazy shrinkage. This helped my project, particularly because I'm using a mixture of linen and cotton which shrink at different rates.
Here's a peak at my pillow I put together for the tutorial using with one confetti block. It's a great way to give tiny piecing a try.