OK, so I just spent the last hour messing around with the Fire-TV.
This is, right now, the leading contender in my view as a Boxee Box replacement.
Side loading the latest nightly build of XBMC Gotham was a snap. For grins I also side loaded the Milestone Xprotect surveillance application that I use. Side-loaded apps don't show up in the main application area, but there is probably a way of building a shortcut somehow so that they show up there.... I will be investigating that shortly.
Performance of the box is impressive... at least as fast as Boxee Box if not faster. Since you have access to full blown XBMC the GUI performance will largely be dictated by which skin you choose... however using the "out of the box" default Confluence skin the performance was impressive. Full blown 1080P fan art, cover flow working well and pretty "snappy" when scanning through my large library of about 500 films.
Playback performance is impressive, with a couple of caveats. I played back a full rip of my blu-ray Thor: The Dark World and it played back flawlessly, including typical XBMC goodness like automatic detection and display of forced subtitles. FF/REW buttons on the FireTV itself aren't working in XBMC yet, but you can do "jumps" with the directional pad and those worked very well. XBMC gets it's marching orders as to display resolution, etc, from the host Android device, so, you can't for example get 1080p/24 working since Android (or the FTV itself) does not support it. 1080P/60 however worked VERY well with everything I tried with it, with a couple of notable exceptions.
1. FTV does not fully support VC1 hardware decode. This is quite a blow to local media fans since SO MUCH GOOD STUFF is encoded in VC1. Probably 80 of my films including many favorites like The Matrix are in VC1. When you play them back with FTV you will get a slide-show. It's unlikely that this will be changing anytime soon. Options are to simply not watch those films on this device or transcode the VC1 titles into the much more universal H.264. Since I only view this as a device for use in secondary rooms of the home I MAY be able to live with this... not sure yet. On the plus side the CPU in the FTV is good enough to decode 480P MPEG-2 DVD without issues... so even though it's not supported in hardware it seems to work just fine.
2. FTV does not support HD audio bitstream. It will down mix True-HD to 2.0 audio and it will extract 5.1 core DTS from DTS-MA. Again, for use in a secondary room not necessarily a deal killer but another limitation that some of us will view as crippling our full enjoyment of local audio.
3. I can't find a way to power it off. The FTV appears to be "always on" until it "goes to sleep". It does have an obnoxiously bright white LED that is lit when it's in use that I would have to cover with tape if I had it in a bedroom as it is really annoying... it's too bad there is no dim or off setting for the LED light in the settings.
4. It is fanless... and tiny... and super low power consumption. All of this is GOOD.
5. It has a BT remote, which, while not having a kb like the Boxee is of slightly better construction quality and has a "voice" search feature that you can use when using the FTV with any of the many streaming services shipping on the box.
All in all this box is MOST impressive. It's $99... has a nice remote, is tiny and uses very low power and ships with an impressive app store. Being able to run XBMC on it is certainly a killer feature from my perspective.
Now, on to the let-downs. This box (and most other boxes like this) are simply not geared for VC1 playback or HD audio. For those uses our choices as media snobs is to either down mix our Blu-rays (blasphemy!) - make second copies of the movies for playback on this device, or simply don't watch those titles on this box. It's disappointing but with BD on the down-spiral towards eventual obliteration it is sadly to be expected.
Now, compared to the $199 that Boxee originally cost or the $250-$300 that a NUC costs to build (which still has issues with VC1 by-the-way) this box is compelling. The ability to side load and run pretty much ANY android app is impressive. The previously mentioned Milestone security app? Well, I had that configured in about 5 minutes and was watching my 2MP camera feeds and the quality was outstanding....
FTV, right now, is definitely the box to watch. There is so much interest in it in XBMC land that there are patches going in to the Gotham code weekly to improve support for Fire TV. For $99 you can almost view it as a disposable toy to play with while you wait for your Boxee Box to kick the bucket.