Internally things are quite cramped, as you'd expetc from an SFF system this size. There's space for two 3.5in drives, one accessible through the front panel and the other a purely internal device. There's a 5.25in drive bay for your optical drive, which is hidden behind a flap on the front of the machine. There's also a single PCI slot and a x16 PCI Express slot for further expansion.
That said, there's no dedicated power connector from the 250W PSU for a PCI Express graphics card, although you could use an adaptor. However, I would go with a slightly different model from Shuttle if I was to use a dedicated graphics card, but it is at least an upgrade option at a later stage.
The SN21G5 is fairly quiet in operation, although this does to some degree depend on your choice of hard drive. There are only two fans: a small one in the PSU and a 92mm fan at the rear that cools the CPU heatpipes as well as the system as it sucks air out through the back of the case.
Building a Shuttle box is quite easy and the SN21G5 doesn't have any particularly unusual features, although a few more pre-routed cables would help. As I used a SATA optical drive I also noticed that there's only two SATA connectors, so if you're using two SATA hard drives, you'll have to use an IDE optical drive with the SN21G5.
Performance-wise the SN21G5 isn't a scorcher, but it performed well for a machine with integrated graphics. It's definitely not a gaming machine, but it didn't have any problems with general office tasks. I tested it with an AMD Athlon 64 FX-60 processor, 1GB of Crucial Ballistix memory and a Western Digital Caviar SE16 hard drive. That's unlikely to be a typical configuration for the SN21G5, but it does show that there won't be any heat issues using slower Athlon 64 processors.
The SN21G5 is a rather basic small form-factor PC with integrated graphics, but it should appeal to anyone that isn't out to build a gaming rig. Or, for that matter, a budget system - you're likely to be able to get a µATX/mATX motherboard and a small case for less than the £220 or so that you can get the SN21G5 for. It's quite a pricey box considering what you get for your money.
Still, a Shuttle box would take up less space and it also looks better than most µATX cases - but add a processor, memory, hard drive and an optical drive and you're close to be able to get a Mac Mini or AOpen MiniPC for the same amount of cash. It's a matter of what you want to spend your money on, although the Shuttle has the advantage of being upgradeable. ®