I just downloaded the trial yesterday, and I can't get it to work.
It's because I have a nonstandard network setup. I didn't want to spend $100 for a wireless adapter for my 360, so I ran a line straight to a nearby computer, and I'm using that computer as a gateway to my wireless network. Unfortunately, doing it this way puts my 360 on a different intranet than my main network, and my 360 can't see the PlayOn host server.
I'm thinking about biting the bullet and running a very long network cable from my router, through my crawlspace, to my Xbox.
PlayOn does look awesome. Hopefully, I'll be able to get my system working with it.
Edit: I ended up turning off internet sharing and I bridged the network adapters instead. PlayOn worked when I did it that way. Pretty cool. I'm not really impressed with the video quality. I wonder if it is dependent at all on the internet speeds.