My favourite accessory for my Bernina (440) is the multi-thread holder. It is just the best for the bigger reels of thread and especially metallics and the silky threads such as the Madeira Polyneons.
Before I had the holder I'd put the larger reels in a small container out the back of the machine so the thread would thread through ok. This mostly worked although there were times when the reel would bounce out of the container or the container (and thread) would end up on the floor.
While there are single thread holders available to have the ability to store several reels is great. If I'm working on a project with multiple thread colours I can have them all in the one place. What is a little annoying is needing to unthread when the bobbin needs to be wound.
The unit comes with one main holder and two smaller holders that can be attached to the back (see photo above) or sides (see box photo).
The best time to purchase one of these multiple thread holders is when the Bernina dealers have their accessory sale.