Yes it works. I got one from the US last week.
You can access any iTunes account with it. (I have both a UK and US iTunes account).
There's currently no TV shows available yet with the UK version of iTunes. I presume this will change over time though.
There's plenty of film rentals with both accounts + TV programs starting at just $0.99 per episode with a US iTunes account.
Very easy to set-up (works with either WiFi or a wired ethernet connection) and link your iTunes, MobileMe, YouTube and Flickr accounts. With MobileMe and Flickr, you can access your online photos.
All you need is a HMDI cable connected to your TV.
Once you enable home sharing, you can access any iTunes account on any PC on your home network and stream both video+music to your TV.
You can use either your iPhone or iPad as a remote control, once you get the free iTunes remote app.
There's also going to be more integration with the iPhone/iPad once iOS 4.2 is released later this year.
Very impressed with mine so far.
It does seem to consume quite a lot of data though as over 2 days, I've used an extra 2.5GB of data, so just something to keep an eye on if your broadband account has a monthly usage limit.
Some online reviews are also knocking it for only having 720p HD output but unless you are using a HD TV bigger than 55" you really wont be able to spot any difference between 720p and 1080p HD output.
Regards
James Mitchell