I use both a Dillon progress press (550) and an “O” frame single stage press. So, my world is a mixture of two systems. The dies for the Dillon are semi-permanently installed in removable toolheads, and rarely get adjusted. Sort of like Jeff’s Lee turret press.
I use a single stage press for rifle cartridges and odd-ball handgun cartridges that I don’t want to set up the Dillon for.
The interrupted thread of the Lee bushing system is a well proven design. I don’t think the system is horrible, in fact, interrupted threads are used in lots of applications. I just don’t think it’s that big of a deal to thread a die in and out of a press. That 4 seconds it would save isn’t worth the money to me.
And like Jeff, I would have to be all in if I converted to that system. The bushings are a one-time expense, but I would need to completely commit to that system across the board.
Like Rick, I worry a little about potential tolerance stacking with all those thread interfaces and flat locking surfaces. It probably would never be an issue, but it would be frustrating and expensive if a problem appeared.
In our hobby, we each pick paths to go down and purchase equipment centered around those paths. For example, if you powder coat, you acquire the gear for that. Or if you use Lee turret presses, you tend to buy lots of turrets depending on the style you need (three-hole or four-hole turrets).
If you go down the path of Lock n Load bushings, you buy a bunch of those.
Maybe if I was just starting out, I might look a little closer at that system.