I've been using the beta now for about 5 months, and in my experience it's cosmetic. Those cosmetics definitely are an improvement in usability and looks.
IMHO, it's a small step, but in the right direction for sure.
I found the beta to be precisely the opposite: it improved a number of functional factors, but aesthetically, the design is a step backwards.
For example, the functional separation of the content tabs (article, discussion) from the tabs representing what you're doing with that content (read, edit, view history) is a good improvement. But the design used to implement that is terrible.
There's no boundary between the top of each tab and the space above it, making it difficult to recognize the tabs as a tab interface at all - it looks like loose text in the middle of some whitespace. And there's no design element that represents the hierarchy of content - the read, edit, and history tabs are now on the right side of the screen, but how would a new user know they are subordinate to the article and discussion respectively?
I think Wikipedia misstepped here. They should have gradually introduced each new design element and proven that it could fit successfully into the structure of the site before building further on the innovations. Releasing an entire top-down redesign all at once is going to create a lot of unnecessary disruption.
IMHO, it's a small step, but in the right direction for sure.