This is the machine that could've rivalled the IBM PC for desktop superiority had Ken Olsen (CEO of Digital) let it; I believe his words were "The future of computing is not on the desktop!", though some sources have a similar quote: "There's no reason for any individual to have a computer in their home". This almost rivals IBM's chairman Thomas Watson's quote of 1945 that he didn't see a need for the world to have any more than 4 computers :)
Beautifully put together and very easy to expand, this machine actually featured twin CPUs - an Intel 8088 to run DOS or CP/M-86 and a Zilog Z80 to run CP/M-80 (thanks to Bill Leary for poiting that out). Not at the same time, obviously, but you could either boot the relevant OS off the included twin RX50 drive or if you'd bought a hard disk machine or upgraded to the Rainbow 100B+ you could split the drive into partitions, and each partition could have its own OS installed! Need to change or add a motherboard component? Simple - 2 catches to lift the top straight off, unplug any drives present then 4 thumbwheels to completely remove the board - no screwdriver required anywhere on this machine. Even the power supply was held in with a retaining catch.
The only thing I don't think you could do to the Rainbow was add an external expansion box.....once you'd got your floppy and hard drives in there that was it, but the drives did get up to 154mb. Eventually.
Remember this was before the time of an industry standard interconnect - yes the IBM 5150 had its 5 slots, but at that time they were simply expansion slots. The term ISA (Industry Standard Architecture) hadn't been coined. All the options (at the time) that you could get for the 5150 were available for or built into the Rainbow.
I got this machine from an old customer (same one I got my VT100 from, actually) and I've only seen one other machine that was dirtier, and that one had been in a fire :) Maybe I should mention that this customer made the glass panels for oven doors, car sunroofs, halogen hobs etc, and this machine was in the production area, so I suppose I should be grateful that it even works! I've totally cleaned the internals, but I think all I can do with the case is tidy it up - the plastic's degraded too much. Shame.