To the original poster...Fast Fire III is trouble free and easy to operate. With Fast Fire II you'll need to remove sight to change batteries. I've never owned one (FF II) but that feature would worry me. Any time you remove a sight, there's the possibility that your point of aim will change.
Otherwise, Fast Fires are durable and reliable. Forever warranty to the original owner and any subsequent ones. No need to register. Just send it to them, no questions asked repair or replacement.
I'd recommend FF III without reservation.