I've been going around and around with myself debating on installing a media server in my HT. Without a doubt, my biggest fear is the degradation of DVD video playback.

So here's my questions:

1. Does anyone have one?
2. What brand?
3. Are you satisfied with its video/audio reproduction compared to your standalone units?
4. What are some of your likes/dislikes?

Personally I just build them using Windows XP or Windows Media Centre edition. I was using Beyond TV for a long time but switched over to MCE and can't say I've been disappointed. I do not play DVDs on it though. A $150 Oppo is a better bet for that.
I think the Slim Devices offering(s) is/are very slick and have been well reviewed. Music only I think.
If you want pretty and excellent performance you might want to look at http://www.stereophile.com/mediaservers/406olive/

If you're interested in good sound make sure that your candidates play lossless codecs and the more video formats supported the better.

A little off-topic, but...

In the "non-standard solution" category, I still recommend considering a modded Xbox running XBMC (the freeware Xbox Media Center software). Although I recognize that this isn't for everybody--particularly if the main purpose is just to archive DVD content to a central storage device--it is still a very handy and affordable solution.

With a large drive installed in the Xbox, this can work quite well as a media server. (Although I don't use it this way. I have it networked to my PC and stream content from that system.) XBMC supports just about every lossy and lossless audio and video codec and sports a very nice (and skinnable) interface.

As ej anticipates in the original post, there is a compromise in DVD playback. I'm still glad to have my HD A1 for that.

PS -- I also understand that the Xbox 360 is good for this if you're running Windows XP Media Center on a source PC; but I haven't popped for any next-gen system, yet.