I've been using it on Ubuntu 8.10 for the past few days and so far I like it. The interface is intuitive and clean, and can be used without a keyboard. Someone even got a wii-mote working with it.
I've currently got 15G of music and 25G of video that it has cataloged so far. I've watched some video on Hulu and CBS, which is nice. If you've got friends in Boxee, you can choose to display the media you've watched/listened to recently, and make recommendations to your friends.
There are flickr and picasaweb plugins to view your shared images as well as search for others users. I use the picasaweb plugin to view my pics and I have no complaints.
You can set up as many media sources as you like including local folders, network shares, and RSS feeds.
That said, there are definitely some issues. Here is what I've found so far:
* Media cataloging doesn't always use the metadata within each file, so you might get some incorrect movie thumbnails (e.g. The Office episode "Halloween" gets a thumbnail from the movie "Halloween") and they aren't sorted into TV shows/movies - you have to do it manually.
* So far, all of my video has been cataloged as a movie, but they are actually television episodes. I'm not sure if the video type saved in ITunes is saved to the file metadata, but I have to manually set each episode to be displayed as a tv episode and I must type in the chow name, season and episode title. It's tedious to say the least.
* If you use it with Ubuntu, there are issues with Pulseaudio that can crash Boxee - there are ways around this though.
* Boxee has crashed on me a few times for no apparent reason, but it starts back up with no problem.
There are definitely some things that need some work, but it's a good start. The cataloging and metadata problems are just frustrating because it can take so much time to correctly catalog everything. If these two things get some attention by the developers, I'll be putting an HTPC box running Boxee in my living room.
I was able to email a developer and get an alpha invite pretty easily. I'll see if I can find it for you.