Brent will likely chime in as he uses them too
I would call them the best hydraulic flat tappet lifter out there for FEs right now, for multiple reasons.
First, it is a real FE lifter, not an SBF, so if you already have pushrods, there is little fear of having the wrong length. Second, although that spinning slot of oil isn't always in the right place to help the lobe, it is there more often than no slot would be, so it can't hurt, and I haven't seen any indications in oil pressure. Finally, they run nice quiet and although I have only used them on 280-ish lobes, I haven't seen any float when combined with good spring setup
That being said, I wouldn't call them a necessity, but I am pretty impressed with them
