The impulses aren't all the way there yet. They're so close that it hurts, but nobody seems to be willing to expand on the technology at this point. I've given up trying to get people on the Nebula bandwagon.
After a few weeks of experimenting with reamping using impulses, I think that they're something that you might be able to get away with for demo/EP level releases, but definitely bring out the real cabs for serious recording. Real cabs have a sort of air and dynamic nature to them that the impulses don't model. The best way to describe the difference is by thinking of impulses of being a static snapshot, whereas cabs are a moving picture. The cabs always sound sweeter and seem to respond to the playing more fluidly, whereas the impulses don't tend to change depending on what's being fed into them.
My best results have come from:
poidaobi's higher presence impulse & metaltastic's impulse pack
I've had some moderate success with the Sperimental pack and GHs impulses. No success with Recabinet yet, despite multiple attempts.