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Computerworld - If you've already virtualized the servers in your data center, desktop virtualization may seem like the next logical step. But businesses are finding that the benefits of hosted virtual desktop technologies are more nuanced. The advantages may be harder to quantify and harder to justify based purely on traditional ROI calculations.

So, how do you calculate and quantify those advantages, choose the right technology and build out a successful hosted virtual desktop infrastructure (VDI)? Computerworld asked consultants, analysts and users who have been there to report on what works, what doesn't and how you can learn from their experiences. The first place to start, they say, is with a clear-eyed understanding of the potential benefits.

The gains you should expect from hosted desktop virtualization projects are very different from what accrues from server virtualization. While server virtualization produces visible savings by consolidating physical server hardware and increasing resource utilization, most shops will find that hosting virtual Windows PCs requires a greenfield build-out of new infrastructure in the data center.

But that hasn't stopped some IT shops from exploring the options.

When it comes to hosted virtual desktops, many organizations are already kicking the tires. "Most of my customers are asking about it, if not going to a proof of concept," says Scott Mayers, a principal director at Align, an IT solutions provider focused on the financial services and retail industries.

"2011 is the year when a lot of those concepts will mature into actual deployments," says Ian Song, an analyst at IDC. But so far, he adds, most deployments are still fairly small-scale. The market research firm projects that only about 13.5 million out of 400 million PC shipments this year will be VDI implementations -- just over 3%. By 2014 that number will more than double, to 34 million, accounting for nearly 7% of the market.

Song expects the trend to eventually top out at about 15% to 18% of all enterprise desktops. Gartner's figures are even more conservative. "While it's a big opportunity, we believe that only 10% to 12% of the installed base of PC users will actually use it over the next two to three years," says Mark Margevicius, an analyst at Gartner. It's a technology that needs to be chosen for the right use cases, he explains.

While VDI is at the top of the hype cycle today, there are many flavors and options. For example, you can choose a "persistent" desktop, where every user gets his own dedicated, fully customizable installation of Windows residing within a hosted virtual machine, or go with the more efficient "nonpersistent" VDI model, in which many users' virtual desktops are spun up from a single, common cookie-cutter disk image.

There is no one-size-fits-all solution. "Every group has its own set of requirements and parameters," so a different mix of technologies may be appropriate for different groups within an organization, says Steve Kaplan, vice president of the data center virtualization practice at infrastructure services provider INX. And for some applications, the technology simply doesn't make sense.

The cost of deployment has been coming down also, although the upfront investment in data center infrastructure is still high. "We don't envision hosted desktops being less expensive than a PC, from a capital investment standpoint," Margevicius says. He puts the total cost at about 1.3 to 1.5 times what IT would pay for a traditional PC deployment. "The initial capital investment is the limiting factor for our clients," he says.

On the plus side, desktop virtualization's benefits include better security, operational efficiencies and faster restoration in the event of a business outage.

Given all that, how do you navigate through the process? Consultants and users recommend a cautious, methodical approach. Here are some considerations as you move from a review of the basic value propositions and potential use cases into pilots and actual deployments.