I've been vocal in the past about Karma's slowness and complexity, but I have to say I've been digging in more recently and am finding lots to like.
I'm working on developing a unique visual style for a big project I have planned and I'm searching for a rendering solution that gives me total artistic control and a smooth workflow. So far Octane has gotten me the closest to what I'm after. But I recently took the plunge, moved my scene over to Solaris and really made an effort to understand advanced Karma shading and rendering.
First things first, you HAVE to use "image" view in the viewport. That's the only way to go. After I figured that out things fell into place and I was able to get a fantastic render with my desired look. This took only 30 minutes on my slow laptop, just about the same as Octane in the same scene.
Honestly, this is very exciting. Karma is still a bit slow and has some bugs, but it's really close to greatness. Can't wait to see what SideFX does with it!