Ok, a nice student project, but probably not a very good workhorse.
I'd also recommend something like a Taig or a Seig, though there are many similar brands out there. They sell secondhand for not much more than the Sienci asking price, and it's not too hard to add your own CNC conversion kit. (Or buy one prebuilt...)
I've had an Axminster for over a decade, have clocked up over 10,000 hours on it (yes, I use it a lot!), and it'll still hit 25micron accuracy all day long. (I make aluminium moulds for short-run plastic injection moulding, so accuracy is critical.)
I've no doubt the Sienci "works", but I wouldn't expect wonders from it. There are plenty of cheap Chinese CNC machines on ebay that would offer similar performance. If you're just wanting to dabble with CNC, maybe start with one of those.
There's some pictures of assembling the Sienci on Instructables:
http://www.instructables.com/id/How-to-Assemble-the-Sienci-Mill-One/Some critical points to note (in addition to those mentioned already in this thread):
- table size is quite small. If you added clamps, your work space will be very limited.
- table has no T-slots - not sure how they clamp the workpiece. Heavy cuts need more than sticky tape. The cheap Chinese CNC machines do at least have a reasonable size bed and clamping slots.
- table may not sit 100% flat. This is a big problem for etching PCBs. You only cut about 100microns deep, so if one end is 0.1mm higher than the other, you'll have problems. (You can always skim it first to level things up.)
- 3D printed parts do not instill confidence.
- the box enclosure will make it hard to align the workpiece. Double-sided PCBs would be tricky. An overhanging spindle (like Taigs have) gives much better access, and larger workpieces can overhang the bed if necessary.
- rigidity is critical if you're trying to machine metal. If you're after their stated 250micron accuracy (which is actually pretty terrible for a CNC machine), the frame and spindle must be rigid to some tens of microns. This is why commercially available machines (again like the Taig) are made from cast-iron.
- no drip tray. You can dry-cut aluminum if using cutting bits above 3mm diameter, but smaller fine detail cuts (I use dental burs!) need cutting fluid. If that gets on the MDF frame - and it will - it'll swell and their goes your accuracy.
- steppers. They don't say, but looks like they're using 4-wire bipolar. I've build a few CNC machines and always use 5,6 or 8-wire unipolar motors. They run smoother and can offer better accuracy. (Ok, you get less torque, but speed isn't everything...)
- electronics is probably OK, but not sure how much current can be delivered to the steppers. My primary system has 3A per coil, and then I have 5:1 reduction gearing to boost the torque and accuracy even further. I doubt this machine would like doing heavy cuts in aluminium or even hardwoods.
- I can't find any pictures of anything made from metal, though I expect they've done some tests. The kickstarter page says 80-100micron depth of cut in aluminium. A good machine should be capable of 1-2mm even with small cutting bits (1-2mm diameter). You'd have to do multiple passes to get anywhere at 0.1mm depths, and then we're back to accuracy issues.
On the plus side, I think it's great that the group are working on an accessible CNC system, but this initial offering has some serious limitations.