I've tried all three options, Traxas, magnets, two versions, magnetic cup and steel ball, steel cup and magnetic ball and my current preferred option which is steel ball in printed cup held together with a spring and Spectra line. See page 4 of my Cherry Pi thread for details. I gave up on Traxas early on as I couldn't seem to get rid of the backlash issues. I know lots of people use them successfully, they just weren't for me. Magnets were great up to a point, as sheepdog says above, they don't hold up well at high speeds. The best I managed on Cherry Pi was 150mm/s infill. After that it was down to luck if they held up for an entire print. The spring and string solution I now use is great. There is absolutely NO movement on the nozzle if you try and move it with a finger. I opted for 8mm drilled ball bearings as they are way lighter than the 10mm magnets I was using and as the cups they sit in are printed into the plastic parts I also saved the weight of the milled bolts I used as cups. My current effector is extremely light. Non travel moves on my machine are now at a reliable 250mm/s with acceleration set to 9000 in Marlin and jerk at 20. I can't yet print higher than 100mm/s as my current extruder can't keep up with a 0.4mm nozzle. Precision at these speeds seems equivalent to 50mm/s speeds with other iterations of delta rod connections I have tried. I use Cura and my printed parts are, in my opinion, excellent. Unless something new hits the scene I will stick with this arrangement.
Andy
Andy