The debugging in SDBG is not yet 1:1 with the classic GWT devmode debugging.
While the Variables and Expressions view do work (make sure you also play with the "Show Logical Structure" button in the Variables view), they expect and show (as of now) JavaScript expressions, not Java expressions.
Further, hovering with the mouse over variable names in the Java editor does not yet show the values of those variables.
Basically, you get more or less the same debugging experience as with Chrome DevTools, but with the added benefit that you don't have to leave your IDE for debugging, you use the same shortcuts as with the serverside debugging etc. etc.
Improvements are planned, but for the future. I also want to see first how the GWT DSM debugging story in Chrome Devtools is shaping out.