This is a legendary barebone PC! I've used shuttles before and generally I found them to be only good for putting a quiet little PC in the lounge as a substitute for a high end DVD player / PVR. This shuttle in particular is absolutely legendary!
Firstly, ALL of its hardware is supported by Mac OSX so you can turn it into a fully functional MAC using a retail copy of MAC OSX 10.5.4 with very little effort. I think this is the only complete barebone PC on the market that is *this* easy to turn into a hackintosh. Beware though, installing MAC OSX on non-apple hardware can void your software license.
Secondly, the Mac novelty wore off so I thought I'll try playing games on this baby. With a £30 CPU, £10 RAM and a 40Gb hard disk I had lying around I now have a new gaming rig that plays my favourite game (Planetside) on medium settings flawlessly. I doubt you will be able to play a brand new high budget title on this baby but if you're into games that are two or three years old (or older) then this is a ridiculously cheap way to get a gaming PC.
The shuttle is so compact that it just stacks on top of a normal PC like an expansion module, if you want a second PC out of sheer greed then it wont require you to re-arrange your house.
The amount of installation required is as minimal as possible, it can be a bit fiddly to fit memory and cables but they've hit a good compromise between space and economy. The only thing missing is a 12cm fan where there is clearly a slot for it, it was a bit tight of them to not give you one in the box.
There is one big catch, it doesn't come with a CD drive bay, or should I say it doesn't come with an opening for a CD drive. There is enough room to fit 2 slimline harddisks and a slimline DVD but slimline DVDs cost a lot and you'd have to permanently remove the front faceplate or get out your jigsaw. For my purposes this is fine, only when reinstalling the operating system do I extend a cable down to my other PC to leech its CD drive but most people will find this to be a massive downfall. You could add a USB CD drive but permanently needing one would wreck the convenience of having something this compact.
If you're tech savvy and want a cheap second PC for a very specific purpose then this is the item of choice!