I've used both and prefer the RokBlokz.
While the Rally Armor flaps themselves are very nice, I'm not at all impressed with the FiST-specific kit hardware. Without having actually dealt with it, you may think that it's all going to be top-notch and well-made, as I did. The flaps and the rear hardware are fine, but the front hardware killed them for me.
1. The front flap hardware is a convoluted and fairly fragile mess. The black brackets/plates that get secured in front of the flaps are made of mild steel or some other potmetal crap and are coated in a very cheap paint. This bracket is in the perfect spot to get blasted by anything and everything flying off your tires, which means that the paint will chip and flake rapidly and the metal will rust quickly. It's a shame because the rest of the hardware is made of a fairly nice stainless steel. I eventually replaced these brackets with some aluminum flat bar stock which held up much better, but they still suffered from getting blasted with pebbles and debris.
2. Getting the fitment of the front flaps to your liking is a nightmare. It's all very fiddly and likes to shift around and bend on its own even after you've secured it. That black bracket is especially troublesome, because it will bend with the flexing of the flaps, leaving unattractive gaps at the bottom between it and the flap, and between the flap and the fender, and will also cause the top to sag forward. Thicker bracket material will leave you with less tire clearance and/or not enough threads for the nuts and won't really solve the inherent design flaw of this setup. At least the bottom gap between the flap and fender can be solved with some high-strength 3M mounting tape between the flaps and side skirt caps.
3. As hinted at in #2, the studs and nuts arrangement of the front flap hardware will bite into your clearance if you're running tires that are at all a larger diameter than stock. No bueno!
4. I'll be blunt: the studs for the front flaps just suck and are a really stupid idea. While adjusting them for the nth time in under a year of use, one of my studs snapped right off. Junk!
5. After contacting Rally Armor customer support several weeks ago about replacement hardware, they have never responded. I decided that I had enough at this point and ordered RokBlokz.
The RokBlokz are much easier to install and get proper fitment. The mounting hardware is much more simple, sits flush and will not bite into your front tire clearance, and they also do not "self-adjust" the way the front Rally Armor flaps did. I do have two things to note about them, however:
1. You'll still get that gap on the bottom of the front flaps without mounting tape. No biggie - at least there isn't an ugly bracket or studs to deal with and the top of the fronts won't sag since they mount directly to the wheel liner.
2. Being made of a stiffer polyurethane, the flaps do not flex like the RA flaps do which is kinda nice but also means that they will scuff/get damaged a bit more easily if they scrape the road.
In the end, you are FAR better off with universal RA flaps or the RokBlokz kit. I believe Jeff has a thread around here about making DIY hardware for universal RA flaps. Just avoid the FiST-specific Rally Armor kit. It's a $150 pain in the ass.