First, let's describe what the Stiffy is. It's simply an additional 50mm linear rail shaft, with an additional roller bearing added to your X rail. It increases the rigidity of the X-axis. It also makes your machine look more impressive, and the same is cool.
I didn't initially buy a Stiffy for my Foreman. One day, I was cutting dog holes through four layers of 3/4" material and I could visibly see my spindle rocking side to side. Now, I was doing something kind of stupid. I was using a 4" down-cut bit which was not properly excavating the chips. Nonetheless, this was the moment I decided I needed a Stiffy.
If you want to cut deep, fast, and especially if you have a 48" X axis, you'll benefit from a Stiffy. I was planning to start using 3/8" bits so that I could cut deeper cuts and faster, so I knew I needed one after seeing this.
Since the upgrade, I haven't seen any flex or rocking like I saw that day before I added the Stiffy, even when using my larger bits and cutting faster than is practical.
Installation is very simple. Onefinity has a great installation video on YouTube. I won't recreate the wheel when they've already done a fine job.