Cantor is quite usable with the new interface. A couple of things do not yet work, for example printing and saving/loading worksheets and using rich text in the text entries. But embedding LaTeX code works, and everything calculation-related should work as well (and this, after all, is what the program is for).
Also there are a lot of minor improvements over the old interface that make using it much more enjoyable, such as more sensible cursor movement and less ways to put the worksheet into unintended states (actually there is none that I am aware of).
But of course not everything is perfect (yet), and I would very much appreciate you to tell me of any bugs you find, or of important features you think are missing. You can find the code here.