We are currently using version 3.8, and it has a familiar look and feel. Version 4.01 is a redesign, making it look more like a website than a forum, introducing more features like a blog, member articles etc.
The only business forum I can find using the new version is the A1 Business Forum. To be fair to them, I think they are still in the process of sorting it out, as it looks like they have header probs, but you get an idea of the functionality.
Me personally, I can't make my mind up. I do like our current style, I think its clear, logical and is what it is, but as always, interested to know what anyone else thinks, or if you're indifferent

I wouldn't rush to upgrade, but if enough people said they preferred the new version, then I'd think about migrating the forum in the next 6 months or so.