I was using Windows Server 2003 R2 x64 Standard Edition for a little over a month as my main developer machine.  I enjoyed it quite a bit, however there were a couple of limitations and annoyances that ultimately made me switch to the x86 edition. 

The main delimma for Windows Server 2003 in general and the x64 in particular is that there are not any wireless cards available for it.  I was in the end able to find a card that works on the x86 build, which made me decide to rollback.  I was doing my best to keep the x64 build but my desk was in an awkward place in our studio loft and needed to be moved.

Aside from wireless support the other reason that I switched was that one day, for no reason at all, SQL Server Management Studio would not launch.  Upon investigation I found that others with x64 builds of XP and Server 2003 experienced the same issue.  I am hoping that this is specific to x64 builds and not a bug in the Windows Server 2003 kernel itself (which XP x64 uses).

Otherwise I enjoyed being able to say I was using a 64-bit operating system, it made me feel advanced in some ways.  I really haven't noticed any performance changes however, from the two modes of processing.  But that is probably expected with my minimal amount of RAM (just at 1gb) and not too great processor (3400).

Another factor for me was when I would look at the processes running and see 32* next to the majority of them.  This indicates that they are running in 32-bit mode, which really makes running a x64 operating system with my hardware and developer useage of it not worthwhile.

In the end if you too are debating between running an x64 build and plan to use the machine more as a workstation than an actual server then I would advice going with either the x86 build of Windows Server 2003 or Windows XP x64.  I know that Vista is just a day away from release, but for my developer needs it is a less ideal solution with this particular machine.