Imho… the very best quality DOF is the one you can achive when rendering. Though, in many cases you’ll need a good deal of AA (especially on a stillframe) to get rid of all the noise. If you can take the renderhit, I vote for using any of the new cameras in conjunction with the new AA to get a really good looking dof. You can also take advantage of LWs interactive mblur/dof preview to get a really fast preview of what the DOF will look like.
AA-settings that will generate a pretty noisefree dof is:
AA: 5
AS: 0.01
Oversample: 0.1
Composited dof is way, way faster, but you really can’t get that crisp image if you aim for very low F-stop value. Also, transparent objects needs some planning if going for composite dof.
When LW9.2 was about to go into open beta, NewTek posted a sneak peak video from a LA usergroup meeting. In that video there was a sequence that showed an animated dof effect where the subject was a transparent glasscube with refractions and caustics. The animated dof-effect looked absoutely stunning and that particular example is a great example where a composited approach would be almost impossible. If I ever find that video I’ll get back here and post a link!
EDIT: Oh and yes… if you can take the renderhit, the new DOF is certanly on the same level as XDOF, but without any of the drawbacks a pixefilter/imagefilter plugin may suffer from.