Same story all around it seems. The HDMI port was a push to allow use of the mini as a HTPC, not to replace traditional monitors.
Re: plex (don't use it myself, but ) Video playback via HDMI looks fine, no worse than any HD channels on my cable channels, but the text and still images are fuzzy.
I made an appointment at the Apple store Genius Bar last weekend, and brought the mini with. They hooked my mini up to the only HDTV they had on hand (a 30-ish inch Panasonic, I think), and the guy said it didn't look so bad to him, despite my pointing out the crap text. He asked/confirmed that I have tried setting the output to 24Hz. They gave the expected line of "it may be 1080 lines of resolution, but it's blown up to 30, 40, 50 etc inches, it's going to lose quality." They're not expecting people to hook the mini up to an HDTV at close range as a monitor, just as an optional output for youtube, hulu, itunes movies, etc, and NOT as a primary desktop monitor. They had no solution for me. I asked if the quality would be any better via the Displayport-VGA dongle, and they did not know, nor had any on hand to test or sell to me. They said I could buy the VGA dongle, and if it doesn't make it any better, return it. Will go back next week and get one, and see if my HDTV has any better quality via the PC input, though I'm not expecting much.
I'm very disappointed, I like the machine itself, but the "bring your own screen, mouse, keyboard, and webcam" was not the way to go. I saw this as a means to gain a large chunk of desktop space so I could run several apps onscreen at a time, and sort through large numbers of photos at a time, but the poor quality of the photos makes it semi- pointless, as I have to move things back to my old macbook to see if the pictures look good or bad). And as for what I spent on the customized upgraded config + the HDTV (has other uses, but this was the primary intention), I could have just bought a monster sized imac or a new powerbook.