openMill is really cool, but it's not the first or the best table-top mill out there. We're huge fans of the MTM projects at MIT (particularly the work of Jonathan Ward, Ilan Moyer, and Nadya Peek), and Othermill, which were great sources of inspiration and ideas, many of which we shamelessly duplicated. The Shapeoko is another great example, and there are tons of other people doing neat DIY CNC projects, each with their own strengths and weaknesses.
We focused on making openMill easy to source parts for, build, and modify - even at the expense of design elegance or performance. You'll almost certainly see parts of this design you want to change - that's the point! We've tried to make it easy for you to make the changes you want.
How well will openMill perform? It depends on how well you build it and how well you use it. If it's well built, lubricated, and aligned and you keep the materials feeds low you should be able to get precision in the .001" range. But if you don't make sure the frame and rails are square and try to mill hard materials too fast then you'll get backlash and chatter.
More generally, even though it may only take you a few hours to assemble the mill, this project isn't really plug and play. Machining is hard, so expect a learning curve before you go from functioning mill to quickly making useful parts. You'll probably break some bits by trying to run bad G-Code. You may not get the axes well aligned on your first try. openMill can't stop you from doing things wrong (including things that will destroy the mill). But the fact that the parts are all cheap makes this a fun way to learn and experiment.
We've also worked hard to keep the costs low and the design robust, so you can make an openMill for about $800. But you should budget some extra money for duplicate parts in case you make a mistake or break something. And once it's working you'll find that you want to buy a few more endmills and some raw material to experiment with. We've tried to make it easy to upgrade. For instance, if you want to add a fancy lead-screw and wear-compensating nut (and boost the project cost by about $200), the parts already include mounting holes for them.
openMill is a high-speed super-sharp rotary cutting tool that moves robotically by itself without regard to the presence of valuable objects (read: fingers) in its path. It can easily put a hole all the way through your iPhone and the hand holding it, throw a metal shaving (or broken bit) clear through your eye, or set material on fire with friction. It also doesn't have shields or safety interlocks. So respect the potential for injury and danger, don't let it run unattended, wear safety glasses, and keep your hands away from it while the spindle is on. Seek adult supervision if you're under, say, 40.