I normally don't use Windows Media Center in Vista, and I had almost forgotten of its existence. I recently tried it out, both from my computer and via a TV and remote.
At first, whenever I tried to view photos I'd get the spinning circle and have to wait a long time. After trying various things, I found this was due to a turned off network scanner. This problem made no sense; why would Media Center access the scanner when I try to access a folder of photos on my hard drive? After I disabled that imaging device in device manager, the delays were gone.
The next annoying and stupid thing I noticed was file browsing. Videos and photos need to be browsed separately and music must be accessed via a database. What a terrible way to dumb down a computer!
When browsing my photos subdirectory for videos, I found that videos in .MP4 and .MOV containers aren't recognized. Both can play properly in Windows Media Player and .MP4 videos play while browsing photos in Windows Photo Gallery. It's as if they forgot about cameras which record videos in those formats! I was pleasantly surprised by one thing however: .MKV videos played.
Overall the interface was acceptable for selecting and playing supported videos. It certainly wasn't efficient even then however. For example, folders I added were put at the end of the list of folders, and they were added to both photos and videos.
Note that I didn't mention a tuner card. That's because I don't have a supported tuner card. For whatever stupid reason, Media Center has stringent requirements and video in devices supported by Vista aren't automatically supported by Media Center.
I guess Windows Media Centre is okay for occasionally using Vista via a TV. However, I can't imagine dedicating a computer to it.