VoP works fine for classes like Monks and Soulknives whose abilities are largely inherent, and have almost no dependence upon material goods.
If you run VoP RAW, its subpar for almost any spellcaster. RAW, it prevents wizards from having spellbooks, and keeps arcane spellcasters from having the material components to many spells on their person. There is language in the BoED that says their are other ways to obtain such components (p30), but several people interpret the VoP to be an absolute bar to ever having such components even for a moment.
RAW, Divine casters lose their divine foci and several class abilities (like Turning).
In another thread (which I can't search for, nor do I have the link anymore), I posted all the PHB spells that would be excised from a VoP caster's repetoir if VoP is run RAW. If you're a religious person, that list would look extremely counterintuitive: essentially, the VoP PC swears fealty to a higher power...who then strips the PC of the ability to affect undead or cast spells that are core to the concept of the duties religious heirarchies owe their followers (like Bless or Attonement).
If you do a slight tweek, and allow arcane casters their spellbooks, divine casters their divine foci, and allow VoP casters to hold valuable material components for short periods of time (about the casting time of the spell), then it should be fine.
Some have reported VoP PCs (mostly Monks) to be über-powerful- but I suspect it is because the VoP's benefits accrue at a set pace that the DM has no control over (if running it RAW) that may be faster than other PCs are accumulating items that would otherwise maintain the balance. IOW, don't use VoP in a campaign if you're planning on being tight with the magical gear- the VoP PC will start to dominate very quickly.