Xeons aren't really the competitors for those, they're replacements for Cavium's existing MIPS64 offerings that end up in filer and network appliances. Apparently (according to a somewhat biased source at Cavium) they're competitive with current Xeons in aggregate performance per Watt, doing better on parallel workloads but less well on single-threaded ones. They really shine on anything I/O-intensive though, due to the integration of the ethernet and SATA controllers on the die (and the design of the DMA engines). They're not likely to be in general-purpose servers, but companies in the same markets as NetApp and Juniper are very interested in them (hence Cavium's investment in getting FreeBSD supported on them).