When used to service conventional forks, the fork leg holder is of limited usefulness.
If you want to loosen or tighten the top cap when the fork is off of the bike, the fork leg holder is indespensible. That's the only use I can think of.
If you need to immobilize the slider to remove a damper rod bolt, you'll still be using a towel and a vice to hold the fork leg. The fork leg holder will only hold the fork tube, it will not fit the fork slider.
When measuring fork oil level, you need to keep the fork fully collapsed, so you'll probably want to rest the bottom of the fork on the floor rather than using this holder.
The design and quality of the fork holder tool is excellent.
My review is based on one use. The overhaul of one pair of forks on my personal bike. If you frequently service forks, this tool would be a nice extra to have in your arsonal of tools. If fork service is a infrequent task that you only perform on your personal bike, loosen and tighten that top cap while the fork tube is still clamped onto the bike.