Thought I'd weigh in with a couple of comments. I think I'd echo what Gary said about Tiki users being more organization-empowering rather than designers. It seems to be my impression that users of Tiki are less concerned with making it look pretty and more concerned about getting the exact functionality they want to achieve as part of their website goals.
From my perspective, Tiki is so large and complex that I'm reluctant to ever stray too far from the defaults, themes included. I've spent many hours in the past customizing themes only for them to break when upgrading to a new major version of Tiki, so I'm extremely wary of doing that again. Instead, I tend to rely on clever use of things like the "Look and Feel" to achieve a distinctive but still default-compatible look.
However, I don't see the complexity of Tiki as a disadvantage, I see it as an opportunity to get the exact functionality I desire, which is why the design/look is always secondary. Sadly, I know this won't jive with many users because first impression is everything and it if doesn't look pretty then many users are already turned off.
I think the themes have come a long way, and Tiki is slowly getting a more polished and professional look. Certainly the 5.1 themes should be more than adequate for most users' needs, particularly with some slight stylistic customizations using built-in mechanisms such as Look and Feel.