For mission critical applications, Storage DRS may be worth the price of admission for some. When a compatible array is used, one can group disk resources as an object, and move the whole object (active disks and all) to another part of the array. This means that aggregated infrastructure can be moved wholesale without outage, as an object, perhaps guided by administratively selected fault detection or just the need for maintenance.
As our Dell Compellent SAN lacked the new drivers, we were unable to perform the heavy lifting promised. You'll need a high-performance SAN transport to move the data around, Fibre Channel at minimum, but other interfaces like InfiniBand ought to do well — especially for disparate or thickly populated array object movement. Protocols like iSCSI (unless over a dedicated and unfettered 10GB link) are unlikely to be useful unless the transaction times will be small (e.g. not much data to move).
Yet at the bottom end of things, VMware's High Availability still works marvelously. Moving VMs from host to host and back and forth from the NOC to the lab worked flawlessly, if somewhat encumbered by aperiodicity in our Comcast transport to the NOC. This trick can now be done by all of VMware's competitors as a basic, but it's part of VMware's DNA and it shows.
From a practical perspective, most of VMware's competitors can do these minimums, but some of the competition suffers from OS version/brand fixation and doesn't have egalitarian support. Others that have egalitarian OS support have weak storage management, and overall virtualized data center/cloud support.
VMware's vSphere covers all of the bases as close to the state of the art as any production software we've seen. It's still wickedly expensive, and it's the one to beat.
Read more about data center in Network World's Data Center section.