I've been experimenting with some CM9/ICS releases, oh I mean "ROMs", with my HTC HD2. I don't think the ICS interface is any quicker. In fact there are some cases where there's a new delay after hitting a button. The version I'm running now uses hardware acceleration for the display so I don't think that's the reason.
The standby power usage appears to be lower. That's great for my phone but I don't know if it's a huge win for a media player unless you're constantly connected to wifi.
There are some nice new features in the browser (browser bookmarks are saved to your Google account for example) and the menu interface is a little more consistent. I like the calendar (it's more like the web version).