I have just upgraded to win vista 64bit ultimate
Aside from the ridiculous update process (we are talking over 1 hour to install sp1 and 2 together) and the nasty " windows update fixes" that actually wreck your computer (i had to reinstall a second and third time because of this) when downloaded 1 at a time, the OS is much smoother and faster than XP imho.
I have a load of programs installed and on XP (SP2/3), once I had this many, load up times were slow and shut down times slow, this was despite heavy duty registry changes to speed it all up.
The main problem with 64 bit is drivers....2 of my hardware will need selling as no bugger will update the drivers now....
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On a side note, I am fed up with Microsoft and its lazy attitude to software. It seems like they just throw out a beta as a main OS and then spend all their time tweaking it, to be confronted with more complaints about how the "fixes" ruin other parts. It is ill thought out.
Not just that, they should have ditched the registry concept years ago, I hate it. Installations should be based on ini and any program I make does so. Install> folder is there. Uninstall> delete folder.
Not> Uninstall, have to use registry editor to get rid of left over crap.
Microsoft have to make an OS which is fundamentally different to the others. It won't be profitable at first but all OS should be 64 bit starting from the next and should ditch the registry completely. If one needs to use older software it will have to be updated or require the use of an emulator.
Sooner or later, the architecture of windows will have to change and the sooner the better