Well part of the issue is if you're running a single VM on a physical box, you may get 80% of the horsepower of the physical box. but what are you really gaining then? :-)
The point is that VM hosting providers run a lot of VMs on shared hardware, which is supposed to bring prices down and keep performance up, but honestly VM pricing still seems pretty damn expensive compared to co-location and also to ISP style web hosting (which is dirt cheap and deals with much of the same scenarios).
+++ Rick ---
>>For other types of apps that same model doesn't work so well unless you have some remote access technology (remote desktop or Citrix etc.) to allow people to access the machines remotely and run on the desktop. I wouldn't think that VMs would be the best fit for this though because virtualization does have a performance impact and you typically want all the hardware you can afford for any kind of large remote access technology. A real physical heavy duty box probably wouldn't be a bad call for this sort of thing...
>
>While we have a beefy physical box running on one subnet a large number of remote data entry VM instances and about a dozen CPU/HD heavy working VM's in another, we guess the perf impact of a VM is becoming borderline - was in desktop between 10..20%, so in better/more direct host surroundings like HyperV should be less. Distributing RAM between host and guest can be finicky ***and*** rewarding, so part of the savings in operator savings have to be invested in that area ;-)
>And we currently stay at XP level for OS, so newer bugs find no nesting place. Will jump to Win8 or stay even longer...