I have all my Neps in a "deluxe style" mix except for N. burbidgea x edwardsiana which is in live LFS. All the neps seem to do well.
The mix I use is approximately equal amounts of:
Smashed Lava rock
For N. villosa, I also added smashed granite chips as recommended by Phil Faulisi.
I have a N. (maxima x fusca) x (spatulata x spectabilis) grown in this mix - now the plant has 10 inch pitchers and it was just a seed in January 2002. It's more likely the robustness of the hybrid than the mix, I imagine. I brought it to the recent NECPS meeting and I will link a photo here once Nathan updates the NECPS site.