It's a very capable bit of open source hardware and it's supported by a highly functional Java client. Together they give you have access to the features of a commercial logic analyser at a fraction of the price. In the UK the board costs just under £50 including VAT. The software is free.
There is one minor pitfall, though. The product's home page has a prominent link to the original SUMP client. That takes you to an old version which is no linger actively maintained - indeed I could not even fire it up.
There is an actively maintained alternative which worked straight out of the box. There are also a number of firmware upgrades. I don't yet know if I need to apply any of these; I'll report when I do.
The sniffer needs at least one accessory: a cable which connects the board headers to the circuit under test.
A single cable gives you 8 connections; if you want to use 16 channels you will need two of them.
You'll also need a USB cable with a micro connector, which is not provided.
The sniffer and cables are available form seedstudios in Hong Kong. In the US you can buy from Sparkfun or the Gadget Factory. In the UK you can find them at SK Pang and ProtoPic.