My point is you don't know what's gonna happen really. Some people have good experiences and some people have bad. This is all down to their environment. You don't have the same environment as everybody else, so I suggest waiting till your project is done if it is critical and then go for it.

My upgrade from X2 to X3 went flawlessly - no issues. 
 
For what it's worth, I also have an old UAD-1 (yes.. "1") card and a few projects that were in various stages of completion - no issues with any of them.

CakeAlexS
 > In what way?  Is it the installation?
 
Well backing up all the partitions... then...
 
The decompacting of the installation files (whatever you do don't run them offline!) takes time, the installation and registration of other plugins away from cakewalk, the rearrangement of plugins, the removal of old plugins, the rescanning of plugin, the troubleshooting of conflicting drivers or plugins (I had that), and cleaning up and defragging.
 
No complaints, just the way it is when you have a major update... but I wish they would have an online store to install this automatically (providing they gave us access to the install setup applications).

So you do a clean install of Sonar and remove all traces of X2?  I just add it to my other versions of Sonar.  I think I still have S7 on this machine. The program itself doesn't use a lot of space. 

Yup I keep the previous Sonar versions, I don't uninstall. More hassle than it's worth. The plugins though are another matter.

My install of X3 was painless. If you keep X2 then just do the custom install and uncheck any of the features in X3 that you already have in X2. BE AWARE! If you ever uninstall X2 you will murder X3 in the process because it will be looking for the plug you had in X2. I did my install this way and it was a matter of minutes I was using X3.