My DP and other VFX colleagues are Nuke users. Since the tool was already at hand, we could use it (with expertise) with no additional investment. This was a huge draw.
Initially, we weren't going to use Nuke for both VFX and grading, until I came across a free plugin that allows us to export an XML from FCP7 (w/handles!), import into NUKE, and write out as many revisions of each shot we want. The same plugin generates a new XML that rebuilds the cut in FCP7 using the latest comp-ed version of each shot. I've round-tripped a few test shots and it works great.
We are presently settling on a workflow that allows us to send out VFX shots to multiple artists (in various cities), and the DP can email his latest grading node to them. When the composite is completed, the Nuke script (not the QT) is emailed back to me to be rendered locally (thus saving the hassle of massive file transfers).
The goal is to use VFX friends from all over the world with as little hassle as possible. There's no single/best way to do all this, so we are still finding our way, but we are learning to do our best with available resources.
Thomas