I keep it as it was in 3.0. The main fighter is still doing a ton of damage. i don't think it has hurt play too much.
I guess in general AC's don't go up as fast as attack bonuses (typically, they kind of plateau out in the mid-20s, don't they?), so I find the power attacking fighter will most often still hit with his first attack. Essentially, what power attacking does is make those second and third attacks a lot less likely to hit. Also, DR is a bit lower then it use to be in 3.0, so a power attack will concievably do more net damage then it used to, even without the change in power attack.
I actually did it more for the PC's then the NPC's. It seemed a little too lethal for my tastes, especially early on.
In addition, it takes away from the added step of multiplying the value by two that goes on in the player's head as he is debating how much to put in, which helps speed up some players a bit.
Just for fun (and I'm not claiming my math as flawless) lets compare a typical 7th level fighter with a 7th level wizard.
7th lvl Fighter: Power Attacking with say a +2 magic greatsword (ignoring potential criticals).
+7 BAB +2 MAgic weapon, and lets say he has managed to get an 18 str at this point, so he's at +4 str.
Attack is +13/+8 (2d6 + 2 mag + 6 (str)= 2d6+8, avg 15 ) without power attack
Attack is +6/+1 (2d6 + 2 mag + 6 (str) + 7 = 2d6+15 avg 22) with 7 power attack in 3.0
Attack is +6/+1 (2d6 + 2 mag + 6 (str) + 14 = 2d6+22 avg 29) with 7 power attack in 3.5.
Now, he still has to hit, and criticals can get the damage ridiculous, easily to the point of forcing massive damage saves (a different issue). Plus he can do this all day long.
I have a player with a 13th level character, high strength, magic Greatsword, plus improved crit. His damage gets up into the 30s regularly. That is with 3.0 rules. We see a massive damage save every few sessions.
A 7th level wizard using a standard area effect damage spell would be doing 7d6 damage (average 24.5), includes a save for half, but can get multiple enemies in an area. And no crits. And gets it a limited number of times a day.
What does all this mean? I don't know, I just throw numbers up there and try and sound like I know what I'm talking about. But anyway, I just think the spells should be doing more damage then the fighter, or should at least be comparabe.
However, maybe the 3.5 change was put in to allow two-handed weapon fighters comparable to two-weapon fighters and sword and boarders. If someone has done it and has proven it has to be that way to make it worth playing a two-handed weapon fighter, I'd switch it back to the 3.5 RAW.