I've been testing GWT Designer with Smart GWT and encountering a number of issues. Unfortunately it's difficult to pin down exactly when they happen and what causes them.
However I think I've pinned down at least one particular problem. I am using a GUI design pattern that involves nesting H/VLayouts hierarchically within other H/VLayouts.
I've found that manually changing the GUI-specified drag-resizing of these layouts to a percentage (so that, e.g., two widgets in an HLayout can take up 33%/67% of the total width) sometimes results in no apparent change in the design layout. Looking at the generated code, I noticed that the, e.g., layout.setSize("25%", "100%"), is occurring far downstream of the object ('layout' in this example) creation. Manually moving this to right after object creation fixes the problem. So this appears to be a case where it is important to do certain operations in a specific order.
A related issue is that it would be extremely useful to be able to specify widths/heights in terms of percentages and '*' (remainder) rather than fixed pixel sizes, since these are logical and permissible values for layouts/widget dimensions.
I understand that this process of interpreting the code/generating the code between the editor and the visual designer must be devilishly difficult, but this appears to be a significant problem.