I did testing with the new VMXNET3 and tested iperf performance between a few different 2k8 and 2k3 guests. I was thoroughly impressed. When moving data vm to vm within a host I was seeing 1.33GBit /sec transfer (roughly 25% faster than the e1000 driver). If communicating to a different guest on a different host it was still 300 to 400 GBit / sec transfer (roughly a 5 - 10% increase in speed). This is on a couple ESXi homebrew machines with an Intel Mobo and Intel pro1000 Nics. Amazing that the 2k8 servers network stack was that much better too, 5 to 10% increase between 2k8 r2 guests than between 2k3 r2 guests.