I've had Flex3C for quite some time, now at WF 3, and I have had a great deal of difficulty learning it as I make my living machining, and it's difficult to find a block of time to get past the basics. I did find a nice block of time over the holidays, and made very good progress. The key to this was learning to use references and datums, something that I will need to get much more proficiency. Once there, much of the other functionality, including assembly, and mechanism seemed to be a much easier task.
I should add that I also have an ACIS CAD application called Concepts Unlimited, which will be familiar to anyone that has used Ashlar-Vellum or Cobalt. It is a very easy to use feature based modeler (not parametric or relational).
One of the engineers I work with often (I am an ME as well) is a Solidworks user, since the 90's, and I was taken aback to find that he had never used surfaces or much beyond the basic functionality, excepting sheetmetal. Doesn't need much for building automated equipment I suppose, but that was my prime connection with anything SW.
Now comes the kicker. I had the opportunity to purchase SolidWorks Office Premium at a discount (which was coincidentally that same as maintenance), which would be useful for supporting iron mold work for a growing customer. Once installed, I ran through a little tutorial, and of course, the similarities with Pro/E are quite apparent, though I found the layout very busy compared to Pro/E, and SW makes many more assumptions that give the illusion of easier use.
I'll have to leave it at that, as I haven't opened SW for about a week, but I will conclude that SW is easier to learn and get a fast start on. Pro/E, on the other hand takes more effort early on, but once learned, will put you at a much higher proficiency level for later development of additional techniques.
Six of one, half a dozen of the other, both are suitable tools. Having gone through my little climb on Pro/E, I find that I am looking forward to getting the time in over the next year to master Flex3C and the other options that I have.