Benefit: If you have been prone since the end of your last turn, the penalty to your Armor Class against melee attacks made against you is reduced to –2. In addition, the bonus to your Armor Class against ranged attacks made against you is increased to +6.
I kind of want to make a fighter who fights from prone.
In order to evaluate feats you have to do against other feats you might take.
Ok. I did:
Slightly situational +1 AC: Dodge, Shield Focus etc.
Moderately situational +2 AC: Reach Defense, Prone Shooter etc.
Quite situational +4 AC: Mobility, Juke etc.
So it's pretty much in line with other AC feats. I wouldn't spend each combat round prone (hence moderately situational), but this feat helps against groups of archers as well as against a big evil caster with deadly rays. And opposed to other feats which rely on foes' actions (like Reach Defense) you can actively pick up options to improve your prone fighting style - which makes Prone Shooter more powerful.
Honestly it will depend on how much control your group has in their adventuring, if your able to plan attacks and move at your own steam then Prone Shooter has more worth...but if your always on the defensive then the situation it can reasonably take effect will be too few to be practical.
It also should be noted of the hidden benefit of being prone, enhanced concealment and cover. What would of been partial cover or concealment while prone may be almost total.
Another way to look at it:
If you want to be like a Lord of the Rings Elf then don't take Prone Shooter.
If you want to be Solid Snake, go for it.