Actually, I have helical couplings on all the other screws. I got the helical couplings for $5 each a few years ago, after I had purchased the Lovejoys. For the Z axis, it seems to work fine. Backlash is less than .005, but I haven't measured it. I zero my Z with incremental jog set to .005, and it moves both diections.
Although the 1/2-10 acme on my Z would show less backlash than a higher lead screw.
I actually swapped it out for a helical coupling, but it wouldn't run with the helical due to resonance, possibly caused by some misalignment. My Xylotex suffers from resonance badly on all the axis, but all the other motors have dampers that get rid of it. The Z axis has a single shaft stepper, so I can't use a damper. Fortuantely. the Lovejoy seems to cure it.
If I had to pay full price for the helicals, I probably would use the Lovejoys, for 1/5 the price. But the new machine I'm designing will be belt drive, so it won't have any.
If you look at the Joe's cnc's at CNC Zone, Joe cut's new, tighter fitting spiders out of Delrin or HDPE to make them virtually backlash free.