After playing mine a bit, I've noticed several things:
1. If you use quarterstaves, you lose out on stunning fist. If you are an unarmed Henshin monk, you lose out on many bonuses Henshin provides for quarterstaves such as increased damage, increased fire/force spellpower, and several Henshin abilities.
2. You'll never have the DC's for your finishers and ki abilities to be competitive in end-game epics. The only way this is even remotely possible is if you take a minimum strength build and totally pump wisdom; I mean like a cleric pump's wisdom, all chips in the pot, no-holds-barred pure wisdom monk.
3. There are no items (other than +wisdom) that can help your monk DC's. The +3 you get from the Henshin tree is just not enough to even warrant getting. There are also no items that are truly monk-friendly that add fire or force spellpower.
So what does this leave you as? I don't see it being anything but a quarterstaff-using monk with a few gimmicky (and that's putting it lightly) fire/force abilities or an excuse to multiclass a rogue acrobat. The tree needs serious work. In many ways, it feels like the dump tree for monks, the same as the rogue mechanic or a protection cleric trees: useful abilities to all rogues/clerics, but nobody serious about rogues/clerics is going to be a full 20 rogue mechanic/protection cleric as the tree lacks anything resembling power on the level of assassin/acrobat or radiant/disciple. A few (interesting) suggestions:
1. Turn Henshin into a true spellcasting monk (like the kind you see in movies and books: a monk breathing fire, levitating off the ground, using kinetic energy, etc). Keep their quarterstaff damage line, by all means, they're still a melee class and need to build ki. Henshin shouldn't be a prestige about a dps class with a few special ki abilities; it should be a monk based on ki abilities with back-up dps with quarterstaves.
2. Increase the Henshin's ability to gain and retain ki, but keep these methods in higher tiers of the tree so that Shintao or Ninjas can't abuse them without heavily speccing into it. 5 uses of meditation is great, but they should regenerate on tier 3 of 'Embrace the Void'; not quickly, but maybe 1 every 3 minutes. The most important and powerful Henshin abilities require a use of meditation -- as they should in fact -- but they severely reduce your mobility, limiting their situational use even further than they already are. Sure, Shintao and Ninjas can get this easily, as well, but what use do they have for it other than gaining ki while waiting for things? Beyond level 12 or so, only Henshin can truly make use of meditation.
3. Give Henshin the ability to use metamagic feats on ki strikes and finishers (and increase ki cost comparatively), or GIVE Henshin ability toggles that work the same way. This should obviously be a tier 5 Henshin ability, preventing Ninjas and Shintao from abusing its uses. This would give some leeway and planning into the build, since tier 5 requires level 12 and prohibits other tier 5 abilities in other trees from being taken. Thus, this would prevent Henshin from being a low level powerhouse (like fire savant sorcerers are right now), but give them better ki usage at the level when ki-based damage abilities begin to wane and decrease in power, improving their performance in this area beyond that of other monks, as they should be.
4. Change the near pointless elemental debuff ki strikes that Henshin has. In the grand scheme of things, these are next to useless and would be better suited to more useful ki-based abilities. It is unfortunate that, in order to get the void vorpal attack, you need to get all 4 of these abilities.