I have used Reaper since about 2012, prior to that I had a hate/hate relationship with both Cakewalk and the Cubase, having said that I believe the root of my previous problems was lack of understanding and less tutorial resources than exist today.
The signup process doesn't appear to be invasive, my guess it will be to encourage an upgrade to Sonar Platinum and also to flog proprietary plugins via marketing emails which can be unsubscribed