FFP are great for fast "tactical" type shooting, if you do a lot of quick holds for drop and wind compensation. Chris is right as you increase magnification the size of the reticle, including thickness, increases. This can obscure your target if it is small enough. I personally use the Vortex Razor which has a .1 mil open center thus eliminating this issues. But their Viper PST FFP does not have this feature and with a .06 mil thick reticle would cover a prairie dog @ about 1000 yards, there again not really an issue for most.
If the extra cost is worth it depends on how much you will use it and of course the bad ass factor