Working as a systems engineer for an SI, having already worked on PLC2, PLC3, PLC5 and PLC5/250 syetems, my first encounter with ControlLogix was in a kick-off meeting for the next project I was assigned to. "oh and by the way, [the client] wants this system done with the new.." (as it was then) "..ControlLogix platform".
There wasn't an immediate opportunity for some training, and although a course was booked, it would arrive too late, so I just had to dive in at the deep end and fathom it out. I hadn't even used any RS software before, we had always used the 6200 series, DOS-based software for our programming. When the training eventually took place, I had already sussed out most of the machine's capabilities and benefits, so it was almost a waste of time.
I have to report that I found it one of the easiest transitions to make, largely because of the familiarity with the pseudo-universal A-B instruction set.
Within a couple of days using ControlLogix, and its programming software, RSLogix5000, I was a convert.
That was about 10 years ago - and nowadays I teach the stuff to maintenance people and programmers. I haven't come across anyone yet who struggled to cross-over from PLC (PLC5, SLC) to PAC (ControlLogix)
The switch from "data-tables" to a tag database I found to be hugely beneficial, I could create data tags as and when needed, and give them meaningful names, saving me the additional task of assigning symbolic references, and even descriptions were mostly unnecessary if the tag-name conveyed the meaning. All good if you lose the project file.
We haven't yet mentioned being able to create structured data (via UDTs) that enable you to store data in a way that is relevant
to the application, not to the PLC. Until you see it, or use it, it is hard to convey the benefits of this.
You say you are experienced with SLC and RSLogix500, I believe you will have a very small learning curve to migrate to ControlLogix and RSLogix5000, it certainly isn't as hard (as Paul B suggested) as learning Siemens.
Enjoy