We have a nice VDI implementation where I work now with 5 physical Citrix presentation servers running Windows 2008 and all the assorted support hardware to make that work. With our environment, it made much more sense to implement VDI since we have a majority of users who are on-site and don’t leave the office, and those who are mobile have web access when not in the office. When we looked at the expense of rolling out new towers / laptops and the associated upkeep costs, we determined it would make much more sense (from a financial point of view) to roll out thin clients and terminal servers running under Citrix. We support roughly 100 end users with them, and with the Citrix setup, they handle the load balancing quite nicely. You'll notice that you will have more control over your environment with VDI, patching is much easier, and overall I think it's reduced our headaches and our costs.
With VDI you can use regular computers which log in to your hosted environment, or you can roll out thin clients. We have both here (most of the secretaries and functionaries use a thin client, and the lawyers get a laptop or tower). One of the big benefits of the thin clients is that we don't care about them getting viruses, the end users can't really download much of anything to the thin clients (no local storage space), and it really forces the people to do everything "in the cloud" where we can make sure it's backed up and that our AV solution is kept current and active. With Terminal servers, in order for most viruses to take hold, the server would need to be in “install” mode. We rarely enable that, and only in extreme off hours to further reduce our exposure. Like anywhere else, you'll still need to keep an eye on how much data people are storing in their directories, but with roaming profiles enabled and a home directory defined on the file server, we don't have issues with huge log on times. The first time someone logs into a given server it will download their profile information, but after that it goes much quicker (just checks to see what's changed from the last time they logged in).
With the thin clients running off flash memory, it's a matter of a minute or two after powering them on until someone can be "useful" and working. One of the major benefits (in my mind) is that since they're working on the servers to start with, should there be a connectivity issue or a network outage (when working from home) we don't normally lose any data. Their data is hosted on the file server and is being backed up as part of our normal backup set, and if their laptop craps out or their thin client fries itself we can simply swap out their hardware and they're back up and working in ~15 minutes (most of which is taken up by my trudging up and down the stairs to their office and swapping out cables). We have had issues in the past with the servers locking up, but with time and a sense of finding obscure hot fixes from Microsoft we've resolved those open file issues and the systems have been rock solid ever since.
As we've upgraded some of the lawyers to newer laptops, we've rolled out the old ones (after a complete refresh) to those users who needed the ability to do things a thin client doesn't allow. If you've got older towers & laptops, there's even a way to load a thin-client like OS (or even Linux for some flavour) to further reduce your risk of exposure to viruses and people seeing how much time they can waste on Facebook.
If you've just dumped a boatload of money on new systems for your users, rolling out VDI probably wouldn't make financial sense for you. If you're looking at upgrading a large chunk of your end users hardware (laptops / towers), then maybe it'd be smarter to invest in some new servers, install the VDI software and see how it runs before upgrading everyone with new towers / laptops. We've seen the occasional glitch (when someone's running a detailed search it slows down everyone else's session on that server) but it's usually short lived and we get very few complaints from our end users now that they understand how the system operates.
This could be a great solution for your business to consider, we will of course advise if it is applicable....