Hi-
I've been using REAPER for about 8 years and stopped using SONAR altogether I guess about a year and a half ago. I started using Cakewalk when it first came out for DOS and upgraded religiously until then. It's not important now why I stopped using it, but this is definitely the end of an era for me and of course so many others. I wish all Cake employees the very best.
The REAPER developers are extremely responsive (witness their recent reversal on whether or not to support ARA) and the forum members are very helpful, much as everyone here has been.
Regarding the colors & general look, I don't personally like the default theme, but there are many user-designed themes & they (like pretty much everything else) can also be tweaked. The flexibility & customizability is really amazing, so if you don't like the way something looks or works, chances are you can change it. Being able to make your own custom actions is another big plus for me.
It does take some getting used to coming from SONAR, but as mentioned, there are excellent videos and the User Guide (written by former SONARite Geoffrey Francis) is outstanding. Edit: Oh, and also the SONAR to REAPER Quickstart Guide:
https://www.reaper.fm/guides/SONARtoREAPER.pdf My $.02-
-Susan