I've upgraded my XPS M1710 laptop and home built media centre PC from MCE 2005 to Vista Ultimate and have been running Vista for a couple of months. There are a few plus points and some definite negatives. I've been tempted to return to MCE 2005 on several occasions but I'm hanging on in there in the hope that I can get used to its foibles and that MS and other suppliers can get their act together and make everything work smoothly.
Pros
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1. There is supposedly better security in Vista than XP. Let's hope that's true.
2. The Media Center application is definitely better in Vista than MCE 2005, especially for slideshows on an HD panel.
3. Prefetching does seem to work and speeds up application launch.
Cons
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1. Windows Explorer is slow and cumbersome to operate. File copies can take ages. This has been well documented but there is no known solution.
2. Lots of apps require repeated acknowledgement that an action was intended (a big fault with the security implementation). e.g. Peerguardian, Shareaza, WinRAR, Treesize, RealVNC. There is no way to permanently disable the prompts other than by disabling UAC completely. That is no solution.
3. Drivers are really dodgy. I've had to go to specialist sites to get up to date video drivers installed. My Bluetooth printer has no Vista drivers and never will have. I can use lower grade drivers but lose the Bluetooth connectivity. As my printer is upstairs and my laptop is downstairs that is a bit of a pain.
4. Aero is an utter waste of time. I use the Vista Basic interface. Aero just sucks up juice. I always work in full screen with my apps. I rarely see the desktop and I really don't much care what it looks like. Dreamscene? Hah! Nobody needs gimmicky eye candy. There is certainly no point paying extra for it.
5. If you want to use Remote Desktop you need Vista Business or Ultimate. That is at odds with the needs of the home user, either functionally or in price terms. RD came for free with MCE 2005.
6. If I wanted the sidebar I could have installed Google widgets/gadgets or whatever they are. My screen real estate is precious, even with 1920*1200 resolution to play with. I don't need it to be consumed by a clock and the weather forecast all day long. What a waste of space!
7. My Colorvision Spyder2 monitor profile gets unset in Vista and I have to reapply the profile manually once booting is completed.
The long and the short of it is that Vista is a bit of a pain in the tits to use but hopefully it is more secure. If your machine comes with Vista then fair enough, you may as well accept it and get used to it. If you're still running XP you may as well stay as you are.