In twelve years of following Web CMS technology, I've found that vendors and open source projects consistently announce two areas of improvement with each new version of their software. The releases typically say something like:
"We've improved the performance!"
"We've improved the usability!"
Performance is a consistently thorny problem. The underlying hardware and software get more efficient each year, but applications get more complicated. As readers of our Web CMS Report evaluations know, performance problems can crop up on the contributor's client, in the authoring interface, publishing streams, and website delivery, as well as several points in between.
Usability is a much-debated challenge. Every vendor declares their tool easy-to-use, and nearly every customer we encounter has usability complaints -- sometimes quite serious ones. We define usability as "fitness to purpose," and therefore essential to evaluate in the context of different personas and scenarios (which is what we do in our research). While there are some clear usability best practices, the fact that vendors create radically different interfaces among them tells you about the lack of consensus here.
So, go ahead and upgrade to the latest version, but performance and usability are a journey, not a destination.