It works best for graphics cards. PCI sound is over rated, but older cards work. The NIC is limited to 10Mbps so are the other ZII cards. The is an OpenPCI USB driver that I have not used, but USB NIC dongles can reach speeds of 100Mbps (on my Deneb for example).
For the cost, it is worth it, but the it takes up a lot of space and may obscure the rest of your ZII/III slots.