I just tried the demo. I used up all 5 tries and I am happy with the results but also confused.
I have a rather slow computer (Core Duo 3.16). When I fly to a busy airport (KDCA for example) with REX on my framerate usually jumps around between 10 and 20. The worst part is the joystick input is usually delayed and it makes it difficult to control a Cessna when trying to land. So I loaded up this Fiber accelerator and within about a minute my controls are responsive and smooth again. However, there is not really any difference to my framerate. That is the confusing part. It kinda feels like I am on a faster computer. The controls are so much smoother and the graphics are decent too. I don't have any kind of good video card so my graphics were never really outstanding anyways.
In any case, I find that this product makes a big difference on my low-end computer. But you'd never know it by looking at the framerate.
Price is a little high for something like this. $20 would be more appropriate. My trail is going to be over in about 30 minutes. So I am going to try to fly back to DC and see how things go without it.