For the last week, I have been testing the Breakthrough Filters Magnetic Filter System. The system uses magnets to attach filters that sit on the end of your lens. The magnetic system means no screwing on and off of filters and no getting filters stuck on adapter rings or on the end of your lens. You do need to screw on the Magnetic filter holder.
Pros of Breakthrough Filters Magnetic Filter System:
Fastest on/off of any filter system
Mount securely but remove easily
Sharp/Extremely high quality
No color cast
Cons of Breakthrough Filters Magnetic Filter System:
Not easily stackable*
Cannot use with a lens hood
*Breakthrough Photography solution to the stackable issue - they sell filters that combine two functions - Their Dark CPL filters combine a circular polarizer with a 3 or 6 stop ND. Alternatively, you have the option of buying an additional filter holder that allows you to use add-on square filters.
Manfrotto offers a similar system that allows you to bring your own filters - they sell you a holder and adapters for your filter. I hope to bring you a hands-on review soon but if you already have existing filters the cost is much cheaper.