Ruskin,
I couldn't tell you what the best brand is, but I can tell you that I have the "Smarty" brand for my Dodge diesel and it works well. Speaking of gas or diesel and which benefits more from reprogramming, I would have to say diesel. Much of this is because of the forced induction (turbo) and the being able to increase that pressure. Yes, most allow you to restore factory settings, I wouldn't buy one that wouldn't. Most are what they call "VIN Locked" meaning that they will only work on one vehicle at a time.
As far a being detuned, that's probably about right. The problem is that we have a bunch of lawyers and politicians setting the emissions standards these days and things are a bit out of whack. Think of an early 90's Geo Metro, those cars achieved upwards of 50 mpg, now think of the modern day Hybrids that can't even get that kind of MPG. Why? emissions regulations of course. Although the end result would be that the Geo Metro would in the end burn less fuel and make us less dependent on foreign oil, the Prius gets the nod although in the end it eats more fuel buy which in a given time period belches out more pollutants into the air. Just like anything else, the gov't got involved and things ain't quite right anymore.
LM