After using SageTV as my main media centre program for a week, I discovered that I didn't miss my regular media centre program (Windows MCE) as much as I had expected I would. SageTV met my media centre needs well enough, and I became accustomed to it without too much trouble. It had a few quirks with the menus and controls, and a couple of barriers that I had to overcome during the initial setup, but Sage TV is a decent media centre application with a lot of potential - especially when combined with the SageTV Client.
That said, there were some minor areas that could use a little work. First, SageTV is very Spartan in appearance; I'm apparently not alone in wishing for a better-looking interface, since SageTV fans have spent the time to skin it for a better look. Second, its menu structure needs to be completely redone; the menu system didn't feel natural, and was difficult to become accustomed to.
Third, it became clear through the review process that SageTV's main focus is television recording and management. The TV options were feature-rich and provided excellent functionality, but the other media centre functions seemed a little like an afterthought. I found the music section to be very underpowered, and although I was happy with the video library, I kept forgetting where it was, because it is labelled "Imported video" and separated from the music and photo menu items.
I also discovered that SageTV would freeze up after resuming from the Windows welcome screen. Since Windows defaults to the welcome screen after displaying the screen saver, I had to restart SageTV every time the screen saver ran until I got around to disabling the welcome screen.
I'm not quite ready to recommend SageTV as the ultimate media centre application, unless you're only looking to build a good DVR for TV recording. However, I will definitely keep an eye on future updates. If, in a future version, they fill in some of the media centre gaps that I outlined above, I'd definitely be tempted to switch to SageTV as my main media centre program. For now, though, I'll just rank it among the top three media centre applications that I have used.