I am very much a glass devotee and have many hundreds of hours on Avidyne and Garmin.
I think the basic system is pretty intuitive and should be well within the grasp of any pilot as long as they are prepard to invest a little time. Using ground based training software is a very good way of going about this and there are some excellent packages for both the G1000 and Avidyne.
It is without doubt a step up to use the sytem confidently for IFR ops - there are a few gotchas and when the pace hots up during a procedure perhaps because the procedure does not progress according to plan it is not the time to be pressing the wrong buttons.
However I think with any system of this sort, their are those who are going to be a lot more comfortable than others. If you use computers all day I suspect you will be in the first group, and if you have a natural fear of technology and have always lived with a conventional six pack I suspect you will find the transition much more difficult.
In my experience the problem is not so much with the displays; I think nearly everyone with a little time gets quickly accustom to the different ways in which the data is presented but the problems arise with understanding how to transition between the various functions.