That's a good question. I haven't noticed a performance improvement, but I haven't tested network throughput thoroughly. I have been able to use the vmxnet2 (enhanced vmxnet) on almost every OS that I run in my environment. The flexible was a little more limited. Another thing to note is that if you don't have the tools installed, then the flexible NIC still gives you a driver that can be used on the network (assuming your OS supports the vlance NIC), while the vmxnet2 NIC will not connect to the network without the vmware tools.

-KjB
VMware vExpert