I think the biggest issue with classes is subclassing, it looks like a good feature to have, but ends up being a problem.

If one avoids subclassing, I think classes can be quite useful as a tool to organize code and to "name" structures. In terms of performance, they offer some good optimizations (hidden class, optimized instantiation), not to mention using the memory profiler when all your objects are just instances of "Object" can be a huge pain.