Power Attack [General]
You can make exceptionally powerful melee attacks.

Prerequisite: Str 13+

Benefit: On your action, before making attack rolls for a round, you may choose to subtract a number from all melee attack rolls and add the same number to all melee damage rolls. This number may not exceed your base attack bonus. The penalty on attacks and bonus on damage applies until your next action.
<font size="2" face="Verdana, Helvetica, sans-serif">This feat is useful at higher levels as opposed to lower levels, as the bonuses come with a penalty. Such penalties are quite a sacrifice at lower levels, when lack of skill, wealth and magic make it more difficult to hit in the first place. As a result, many playters feel it is a waste to take this at low levels. More on that further in.

Rememeber that this bonus/penalty is limited to your base attack bonus. That number is based on class and level, and not other adds. You may not take a -5/+5 at 1st level, even if you managed to get your hands on a +5 sord and had a strength of 20! This is to prevent low level characters from abusing this power at low levels.

The best time to use this feat, is when the opponent has an AC that is no problem to hit. You never know what the AC of an opponent is for sure, but you can make educated guesses. If the opponent has no shield, very light armor, is awkward and slow, and you have hit him three rounds in a row by rolling a 20, 18 and 15, you can get an inkling that this guy is not buffed. A Power attack makes a great finishing blow! On the other hand, if you are missing most of the time, as your blade clangs off his dweamored plate mail, you might not want to throw your chances of a hit by taking penalties, only to add to damage that will never arrive.

Also note that this bonus/penalty applies to all attacks made in the combat round. If your character can strike three times, then all three attacks benifit from the same bonus and suffer the same penalty. You may not change them around for each stike.

Now back to the part about Power Attack at low levels. To me, the best part of Power Attack is not the actual Power Attack, but the fact that it is the prerequisite to all the best melee feats. Cleave, Improved Bull Rush, Sunder and Great Cleave, as well as feats in books other than the core rules, are all dependant upon the character having Power Attack. In fact, a human fighter can take all of the first three at first level!

I will grant you that Sunder and Improved Bull Rush are not the greatest feats. They pretty much just allow you to do standard fighting options (which are awkward, and slow game play) without taking an attack of opportunity. But as Aoborr, Einkil and Garrick can tell you, there aren't many featrs better than Cleave and Great Cleave!