The 2 main pros to a shackle reversal is the axle path now moves rearward as you hit a bump rather then the axle wanting to move forward into the bump (the reason I almost considered it for my roxor). The second pro is there is potential to gain approach angle when you have springs made the allow for such. unfortunately shackle reversal retrofit kits don't normally have this advantage because to maintain the caster angle they have a big ridged bracket in place of the front shackles.
As for cons, there is a reason that shackle reversal isn't the standard on just about any production vehicle past or present. The main flaws being that the fixed side of the spring being at the font means the amount of drive shaft yoke movement is greatly increased. You loose a great deal of stability on descents as your axle want to roll underneath the vehicle when all of the weight of the vehicle is pushing down on it. This is also the case when braking. As the weight shifts forward, the front of the truck compresses and dives. All of these are exaggerated more with the more arch/travel your springs have.
That being said, it works pretty well on flat springs and slower speeds. The On the Rox Reversal is probably the best riding suspension option next to the long travel kit but it just isn't going to give you the performance that my set up will provide. I certainly would not suggest running a long travel spring with a shackle reversal.
I took the perfect video of a friends jeep to help visualize what I'm talking about. Watch as the axle moves backwards causing the jeep to become unstable and the rear to lift. Once it gets to a point the tire binds at the back of the wheel well making it even harder to drive out of it. This jeep has also broken 2 axle shafts because of tire stuffing.