There is a freely downloadable demo version of Sonar, I believe in its 3 flavors, that you could try out for yourself, prior to purchase.
The audio engine is indeed fully functional - you wouldn't get access to most of the plugins, but you would be able to get a real good feel for the interface.
There have been AMAZING changes to the user interface, and most other pieces of Sonar, as well, and while it might well be quite confusing at first, if you were to not so much try to make it function exactly as 8.5.3 did but instead approached it with a mindset of looking at those changes with more of an open mind - it would help you to get a better feel for doing things.
8.5.3 was amazingly powerful, through all kinds of key bindings and such, whereas they made an attempt to make Sonar easier for newcomers to learn, and that threw a bunch of folks off when X1 first came out with its redesigned user interface. A lot of those same folks have ended up expressing that they came to appreciate the work flow differences - in many ways many folks found the new ways helped them accomplish things faster and better. (this is, of course, not universally shared sentiment, but then again, there is no reason not to be able to continue to use 8.5.3 AND also have Sonar Platinum)
DO give some thought to trying out the demo - it is free, after all.
Bob Bone