I recently switched to Sonar from Cubase (longtime user). I bought Sonar 6 PE. The impetus for the switch was:
1.) I bought a custom built audio computer and was at a crossroads as to whether or not to upgrade to the new Cubase 4, due to increasing concerns about dongles degrading performance, stability issues etc etc.
2.) Steinberg's "customer service" (I use that term loosely) leaves something to be desired. I was increasingly unhappy with the direction the company was going in.
3.) Sonar's native VST support (a must have for me - wasn't keen on the whole wrapper thing in previous versions).
4.) Cakewalk's stellar support/updates/upgrade cycle.
5.) Attractive crossgrade offer.
As a longtime Cubase user, I wasn't happy about feeling the need to switch and learn a whole new host program, but for the most part, Sonar has been reasonably easy to get a handle on. Still can't work out why one can't just double click the name of the tracks and change them though. Inserting soft synths, freeze and adding effects is much less convoluted than in Cubase, but midi editing could stand some improvements, as has been enumerated countless times in this forum.