To be honest, i would prefer to go through the basic/formal training which cover most stuff (at least from interface, sketch up to asm level) at the minimum…
Please dont expect people to learn on the job while still maintaining the same efficiency when they have to figure out even the basic interface 
Also, in general do you find it easy or difficult to learn new systems?
I can provide my 2 cents on this…
So… 1.5 year ago, i started a project that use CREO with close to zero experience on using CREO. At the same time, i am already proficient enough in SOLIDWORKS.
The project have a pretty tight schedule… and you guess it, I do not have much time to go through the training.
I am also too naïve to think that “how hard can it be.”..
Let’s just say… learning CREO on the job is not a pleasant experience.
→ Confusing interface, steep learning curve, very bad system colour scheme
→ A lot of “basic” feature/function that SOLIDWORKS has will take like a 10 steps workaround…
→ Help file is full of words and it not helpful (Gosh, have you seen CREO help file on SWEEP?) , not to mention a lot of time google search will not bring you to the help website of the correct system
→ Want to create a auto-numbering note? Do it manually
→ Want to indent your note? Do it manually
→ Want to create multiple tap hole? You need at least 3 feature
→ Want to isolate parts in asm? Hide manually
And many many many more
What happen in the end?
Well, lets just say we end up with a lot of move face, offset face feature 
I had been using CREO almost non stop for 1.5 year now and had cursed enough to know my way around…
But whether i like it or not is another story
To sum it up, how easy is it to learn a new cad system? That’s depend on your system, whether do you have time to go through a proper training, and how much “past” experience you have with other cad system.
At least for my case, learning CREO is not easy (compared to SOLIDWORKS) and filled with daily cursing