I recently completed my first 8-layer PCB design, for Michael Ossmann's [Daisho project][daisho-annouce]. Among the challenges: a 780-pin Altera Cyclone IV FPGA, a USB 3.0 chip with 0.8mm ball spacing, very high-speed parallel and serial buses, tight power constraints, and keeping the cost down. Oh, and did I mention I was using only open-source schematic and PCB tools (KiCAD)?
It's back from manufacturing, and the first few subsystems are being assembled. Any bets on whether it'll work the first time? :-)