I have an '06 FFV. No problems at all. Drivability isn't a problem. I run 87 octane gas, E85 and mixtures of both. You can tell no difference in power. I get about 20-25% less MPG on straight E85 though. It evens out the fuel savings but at least the Arabs aren't making much, if anything off of it.
It's a nice option to have. Running E85 now and then keeps the valves clean at least. It's a no cost option on the 5.4, so might as well get it.