Win Server 2008 + Desktop Experience
As CTO & professional developer for a software company I ran Win 2003 Server for years as it gave me access to all the server features such as IIS6 that our software runs on.
I moved to Longhorn and to Visual Studio 2008 as my dev platform which was a bit unstable and seemed quite sluggish sometimes - WiFi regularly crashed the machine for example so I only used it when I had to. Aero was pretty cool.
I've recently rebuilt the machine with the release version of Server 2008 and VS08 and have to say that its brilliant. Its faster than Win2k3 and the Aero UI is great to work with. WiFi works now and its faster than Longhorn.
All in all - its by far the best working environment I've ever used (I use a Dell M6300 laptop which is also pretty good).
Big thumbs up from me!!