The current ability to compute values is not quite the same as formulas in a spreadsheet (which hide "behind" cells), this is meant more as a visual programming language where the operators are part of your data.

Example: http://strlen.com/treesheets/docs/images/screenshots/screens...

But yes, I must admit, that part is very unfinished.

There's a preliminary support for formulas with only 5 functions or so, so it's not a fully functional spreadsheet yet.