MotionMile presents a walkthrough showing how you can have particles form type in after effects. The quick-tip uses Trapcode Particular (of course) and covers some really basic techniques for working with the particle / type settings.
Animation becomes simple, exchanging a particle layer for a type layer in 3D space, giving the illusion that they are forming the type. It is a super-simple, but effective way to have particles form type, or any other shape. Although you could probably use any of the particle generators that ship with After Effects, the key is they would have to have the ability to emit particles from an image, mask or layer to function the same way.
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