I have tried a couple of builds using the Kensei Enhancement One With The Blade and have been extremely disappointed. Not your run-of-the-mill "meh" type of disappointment, but more like a "why is it even there wasting space in this tree?" disappointment. As a refresher, to qualify for 1WTB, one needs Weapon Focus, Weapons Specialization, Greater Weapon Specialization (along with Ftr 8), and two branches in the Kensei tree for a minimum of 11 Enhancements. If I am not mistaken, this is one of the most complex set of requirements in the game to achieve a single Enhancement. By comparison, Empower Spell, Maximize Spell, and a Spell Like Ability is far easier to achieve, can be obtained before level 2, and that combination is far, far, far more effective.
A 12 TWF Monk-Fighter using scimitars and Whirlwind should be doing a decent amount of damage - one might think. Minimum level 12, with 8 levels of Fighter and a long list of requisite Feats and Enhancements to get there. In the end, a TWF 9 Monk (or 7 Mnk-Ftr) using an Unarmed Whirlwind attack will kick-out not just more damage, but a LOT more damage, often times nearly killing opponents with a single Unarmed Whirlwind strike. By comparison, the TWF Monk-Fighter using scimitars and Whirlwind is just annoying - as in "stop that, it tickles!" annoying.
[Admittedly, I have not played much in levels above 20, so perhaps this Enhancement has value there?]
Ok. There is a certain coolness factor for a Monk using scimitars or other choice melee weapons. As such, some of the game features are set so you have to sacrifice something (such as one or two Action Points or a Feat) in order to get that coolness factor. But with 1WTB we are not talking about a minor sacrifice for coolness. As pointed out, there are 11 required Enhancements, 3 required Feats, minimum Level 12, minimum Ftr 8, and 23 Action Points spent in a single tree. Whew!
Now, one might ask the obvious question: Why get 1WTB if it's so bad? The answer is because it was there and I thought, 'perhaps with so many requisites it must be pretty darn cool.' My thinking was that like the Unarmed Whirlwind attack, the 1WTB Whirlwind strike would hit multiple times. Turns out, 1WTB + Whirlwind doesn't seem to do that like an Unarmed Whirlwind does. Perhaps the Unarmed Whirlwind is broken, but I would certainly hope not seeing as it requires four Feats to obtain (Combat Expertise, Dodge, Mobility, and Spring Attack) along with 4 BAB. I'd rather like to think 1WTB is broken in this regard, or at least not working as intended and will hopefully get fixed.
Perhaps 1WTB doesn't work well with Whirlwind? That might be the case, but I'll guess "no." The reason for this guess is that Whirlwind multiplies the damage by 4 and attacks in a circle around the character, similar to a Cleave attack. At lower damage (such as damage not multiplied by 4), the results would be the same, however it just would take longer to notice that 1WTB isn't as effective as a Monk's Unarmed attack. If correct, this leaves 1WTB still not anywhere near as effective as a Monk's Unarmed attacks and a lot of requirements to get... role playing coolness?
Why is the 1WTB Enhancement sitting up there at the tip-top of the Kensei tree with so many requisite Enhancements and Feats and then turns out to be pretty much useless? More than useless actually, it ruins any build that takes it (imo). Perhaps a better Enhancement should take its place? After all, its place as the King of Requirements is like Jeoffrey's claim to the Iron Throne (I am a season behind, but I know... I know...).
Suggestions:
Allow 1WTB to work the same as the Unarmed Monk attack in all respects. Remember, this is (likely) the single most complex chain of requirements for an Enhancement, so this isn't over-powered by any means.
Move 1WTB down to a lower (3rd?) Kensei tier without any requirements except Kensei Focus and perhaps Weapon Meditation. Then it becomes a coolness Enhancement.
Allow 1WTB to act as if the Monk was Unarmed. It would be a significant improvement even if the weapon does Unarmed base damage with Weapon Effects added in. This may not be deserving as the King of Requirements, but it would be an improvement.