You're going to need to do a proper cost/benefit analysis on this, tailored to your specific scenario.
The main advantage of implementing a VDI solution is for situations where you do precisely the same thing on a lot of workstations for a lot of users, and can connect them all back to a single central system.
In your case, I'd look very closely at:
- How varied the requirements are between each of your users in terms of the configuration of their workstation, and whether there are clumps of users you could comfortably group as running identical setups
- How much it'll cost you per month to lease thin-client units as opposed to the full workstations you currently lease
- How much RAM per client you'll need in your VDI, and translate that to how much RAM you'll have to stack your host servers with.
- Identify any rich content requirements on clients and be aware of the limitations of VDI that are otherwise trivial to regular workstations. Any 3D output isn't going to be happening yet (Although RemoteFX is available for Hyper-V, it's not gonna be cheap to implement once you start speccing up 4Gb Quadros) and even things like rendering flash or video can be big problems for any kind of remote desktop.
IMO the VDI/Thin client architecture still doesn't stack up as particularly cost-effective, and won't unless the cost of thin client hardware drops significantly. The main benefit is for situations (e.g. Point of sale) where you want to keep the operation of the system as close to the centre as possible).
One option I often float and implement cheaply/effectively is to setup a terminal services environment that serves up the core business apps to any and all clients - This ignores the stuff that people like to customize (development environments etc) but standardizes access for your 'cookie cutter' applications (e.g. timesheet system, payroll). This, and/or shifting some stuff out to SaaS applications (e.g. openair.com) can cut down the amount of extra hassle you have to go through for giving VPN users access (you just let them reach the TS), and less time spent configuring each PC with all the fiddly apps.