iFixit bought one of Apple's tiny but powerful new desktops, then took it apart.
For being so compact, the design is surprisingly modular and easy to disassemble. Non-proprietary Torx screws are used throughout, and several components can be replaced independently. ... While it will require a bit of digging, the CPU is user-replaceable—meaning intrepid fixers should be able to save considerably by upgrading from the base-level processor configuration.
It gets a repairability score of 8/10. With the $3000+ price tag and some proprietary bits, you'll need to have some balls to do much more than snap in some RAM. But you can, if you want to.