In most ways a typical level 6 Fighter NPC, straight out of DMG.
A. Buser: Male human Ftr6; CR 6; Medium-size humanoid (human); HD 6d10+; hp 49; Init +1 ; Spd 20ft,; AC 20 (touch 11, flat-footed 19); Atk +11/+6 melee (1d10+7/19-20, +1 bastard sword) or +9/+4 melee (1d10+6, +1 bastard sword) and +8 melee (1d4+1/20, mwk. spiked gauntlet); SV Fort +8, Ref +4, Will +4; AL N; Str 16, Dex 13, Con 14, Int 10, Wis 12, Cha 8.
Skills and Feats: Climb +7, Jump +7, Prof (Weapon Master) +5. Exotic Weapon (Bastard Sword), Weapon Focus (Bastard Sword), Ambidexterity, Two-Weapon Fighting, Power Attack, Weapon Specialization (Bastard Sword), Cleave, Hand-and-a-Half Fighting.
Possessions: +1 Full Plate, +1 Bastard Sword, Mwk Spiked Gauntlet, Potion of Cure Moderate Wounds, Potion of Fly, Cloak of Resistance +1. (Due to his position as Weapon Master of a personal household, A. has slightly better equipment than a regular npc of his level).
-First round-
Charge and attack with 2-h bastard sword +13 (1d10+7/19-20, +1 bastard sword) [-2 AC]
-Second round-
Full round of attacks, using Hand-and-a-Half Fighting. +13/+8 (1d10+6, +1 bastard sword) and +10 (1d4+1/20, mwk. spiked gauntlet)
-Third round-
2 attacks with 2-h bastard sword, using Hand-and-a-Half Fighting. +13/+8 (1d10+7/19-20, +1 bastard sword)
And it would cycle from there. With access to power attack that feat could easily be as good as weapon specialization. Also note that the only classes that he would not get the benefit against are the 1/1 BAB classes.
I did that mostly for fun, Im sure someone will correct me if I got anything wrong, but as you pointed out, without taking 2-wf into account your feat is pretty powerful. At higher levels he could take some spiked gauntlet feats, and Im fairly sure he could add an enchanted buckler.
Thats sort of the fighter I always thought of, switching the bastard sword around as its needed, your feat just spurred me into thinking of it
Technik
Edited: Making this char a half-orc and losing cleave (and 1 of his skills) makes him equally viable and more of a challenge for PCs (the str bonus gives him +1 damage +1 to hit on all attacks (and +1 more damage when 2-h)). I think it also works as a dwarf (with dwarven waraxe), but his speed is pretty low at that point.