I'm running a temporary 60-day copy of Standard Server 2008 as a pure desktop OS --- which takes a little fiddling with the Roles, Features, Services, and other configuration stuff to acheive (only slightly harder than in the 2k3) --- and the result is fairly acceptable, but I can't see myself getting over-excited about it.
To start with, and unlike the XP SP2 and the 2k3, Server 2008 is still just another intermediary version of Windows --- the Windows I'm personally looking forward to, now that the full release of Vista SP1 Gold is imminent, is Windows 7, which hopefully will be the
real "Vista without the bloat" ... (or rather that the inevitable bloat will no longer interfere with desktop playback speed

) --- as well as the "real Longhorn we've been waiting for all this time"
Nevertheless, there are several things I do quite like about the 2k8
---
I installed the 2k8 Standard x64 without a hitch, as an upgrade of the final Beta build. I was initially a little confused, as some of the GUI is new to the RTM, but mostly the interface is cleaner and easier than Beta was, and in some cases has been rolled back to a more 2k3-like feel, so the end customer probably won't share this confusion with me, provided he's familiar with both Server 2k3 and Vista
Out of the Box, it's very fast, as it uses a new OS architecture which is tending to what we will likely see when
MinWin, the new Windows 7 ultra-tiny ultra-tight kernel, will be implemented --- the OS components are therefore a lot more modular than any Windows version seen since before the W95, so that it is possible to tailor the OS for your real needs, secure in the knowledge that the unused roles and features will NOT hum away quietly in the background sapping your system of resources and speed ...
... however this is still not the true MinWin experience, because the features and roles that you DO choose to install will in fact be mostly re-integrated into your personalised OS core (instead of being fully modular), so that an occasionally used feature may still use its personally allocated system resources all of the time...
Of course, the desktop experience is where this is the most noticeable --- from a sheer performance point of view, the fact that there is no Sidebar clearly speeds up the desktop, because most of the performance wibbles in Vista can be traced to the imperfect integration of Desktop and Taskbar (also imperfect in XP/2k3 BTW) and Sidebar within the WDM ; OTOH because much of the Desktop Experience in 2k8 is optional, if you decide to run Desktop Experience as full-featured as possible, Desktop can occasionally be slower to respond than Vista, presumably because the non-integration of the relevant Services, Roles and Features into the system core must mean that these are run in a sub-system, meaning that desktop responsiveness is occasionally less fluid than could be desired ---- OT3H this is a Server OS, NOT designed to be used as a pure Client version
Desktop seems to be faster, but jumpier than Vista, in other words...
I haven't had any issues with x64 drivers signature since deciding that my old motherboard simply wasn't fully Vista-compatible, and upgrading to a new 2008-era AM2+ system ; having said that, these issues had mostly disappeared during my occasional attempts to get Vista x64 installed on my old system, my issues were elsewhere --- and I cannot find it in me to get excited about x64 unsigned drivers any more, I've just moved on from XP/2k3 to the new timeframe...
Nevertheless, in my current recommendations to people, I do systematically suggest now that
the new Vista and post-Vista Windows versions simply shouldn't be installed on XP-era hardware, well not unless the person is able and willing to handle a certain level of setup pain, maintenance work, and whatnot ; and especially in cases where that person owns some suspiciously "old" hardware
The 2k8 issue that I have had though is that most of the Vista drivers that were provided on the CD that came with the mobo do NOT install correctly in the 2k8 (luckily, alternatives existed), I mean the setup programs didn't even start up !!
A couple of old XP-era utilities that DO work in Vista don't like the 2k8, but nothing drastic.
Anyways, in both areas of Vista-happy stuff being unhappy in 2k8, the setup software, whether drivers or appz, was probably prevented from working because these programs were attempting to change the system core itself --- but I really would have liked a more verbose installation failure instead of just waiting for nothing to happen (and it's a shame that my Server 2k8-compatible system was only ready for the last week of the Beta

)
---
CPU and RAM usage hover at only slightly lower levels than my Vista x64 with my typical software installed and my usual multi-multi-multi-tabs Opera 9.5 up and running, and as noted the less-integrated Desktop Experience leads to a more sluggish desktop than the Vista ------- so that this will NOT become my main OS, particularly as I miss my Sidebar, Games, etc

--- OTOH I am NOT a hardcore gamer, and it is quite possible that the 2k8 could potentially be set up to be noticeably superior for hardcore gaming than the Vista

; and personally I would be very interested to hear about people's gaming experiences in the 2k8
---
So, definitely a mixed impression here --- but clearly, if there were no Vista, this is the OS that I would want
OTOH I can understand that this might be a good OS for some of you Vista-haters out there
XP-Lover --------->
<--------- Vista
... but this might still be another weird one from you guys' POV...
XP-Lover --------->
<--------- 2k8
---
(
Officially looking forward to the first W7 release
)