I built cpu in an computer architecture class and it was one of the most useful things I have ever done, in terms of information learned. It was of course extremely limited (4 bit, so only 16 instructions, 16 type of instructions, 64 bytes of ram, only output was 4 leds for the single register, etc) but a great experience.
we started at the transistor level, building AND/OR gates. Then XOR/NAND etc, then a simple ALU (using real nand/and/or etc chips) then a simple register, etc all the way up, each step getting an integrated chip to replace what we had built with smaller components in the previous step.
A hot piece of iron? Luxury. We lived for three months in a paper bag in a septic tank. We used to have to get up at six in the morning, clean the paper bag, eat a crust of stale bread, go to work down t' mill, fourteen hours a day, week-in week-out, for sixpence a week, and when we got home our Dad would thrash us to sleep wi' his belt. </python>
He once built his own computer, out of nothing but TTL...
A picture should now appear with him surrounded by beautiful women and Dos Equis. (Then Chuck Norris happens by, pauses, shakes his head, and mutters "Damn, that's cool.")