I have been running Plex since version 8. I have it on a Mac Mini in my home theater and it works great. I use the Aeon Skin which is very nice. I also run it on a JB Apple TV2 now in the bedroom since yesterday and it runs quite well. It runs as a front end and streams from either my Mini which runs PMS or the iMac which also runs PMS.
I have tried XBMC and Boxee but always come back to Plex. I do wish Plex ran on the 1st generation Apple TV but I believe it was a choice for the dev's not to get it running on it.
It runs great with media from my NAS (Acer Easystore) running WHS.
Love Plex as a media player. I'm also reading my drives through an Extreme N router. Sometimes I connect my MBP to my LCD TV. I recently helped a family member hookup XBMC on his laptop-TV he's happy with it but I prefer PLEX for the Media Manager.
I also use the PLEX app on my iPod. The app works as advertised.
I use Plex on a Mini and an Acer WHS box. I run the server and client on the mini. The WHS serves up 4 shares (Movies, Music Concerts, Kids Movies and TV Shows). It has been running quite well after tweaking things to my liking. On my iMac I can update the mini by going to the mini IP address and the plex homepage it serves. I find sometimes the automatic updating does not occur after adding files to my WHS.
I have not updated to myPlex yet as I have no need for it. I also use the Logitech Harmony One with it and it works very well. I have used the Apple remote in the past so you should have no problems Hugh.