Amazon have the Fire TV Stick on pre-order offer to Amazon Prime members for £19 delivered.

Saw that this morning, I've currently got XBMC running on a raspberry pi 2 and a chromecast, not sure if I need the amazon tv or not?

I ordered one for the upstairs TV compared to the Roku stick and Chromecast it is way ahead with more memory faster processor. It uses the memory to pre-cache films you are looking at on the menu as a result like the set top box version it starts playing immediately you select and buffering is a thing of the past, the only real downside is Amazon try push their own streaming service and setting a PIN is essential to prevent inadvertent purchases. Having owned Chromecast, Roku, NowTV and a generic Android stick I reckon my existing Fire TV set top box is the dogs danglers.
Roku are also pretty good too I have a Roku 1 in the kitchen which has more or less features as the Roku stick but because Sky/NowTV have a financial interest in Roku they have blocked access to Amazon Instant in Europe.