There is most definitely a learning curve - many folks expressed frustration at the UI redesign the X-series introduced, but many of those very same folks came back later to indicate that they came to really like the new design.
8.5.3 was extraordinarily powerful - But X3e is far more accessible to a much wider audience, in addition to being very stable.
Hang in there - you might consider looking through some of the freely available videos on the new features in X3 - Cake TV and YouTube videos abound. Or, you could look at picking up one of the books on Sonar 3 that are out there - they are REALLY well written. Either approach, or doing both, would really be a good investment of time, and would shorten your transition period, in addition to likely reducing your frustration level.
There are also commercial video tutorials available. I would suggest looking at the free videos first, and going from there. :)
Bob Bone