When I first heard of GSS I thought it was the solution to layout that we'd all been waiting for. Unfortunately it's not all roses. It's possible, and quite easy to inadvertently create cyclical constraint dependencies. Depending on how your constraint solver is written this might mean that a layout will fail or that your system will lock or crash.
I'm subscribed to a couple of constraint solver mailing lists. A common question is "why is my constraint solver doing this...?", or "how do I massage my constraints to achieve this...?"
With CSS You can't build a layout that 'fails' (in the sense that a solution cannot be computed due to improperly defined dependencies), and you certainly can't send your browser into an infinite loop by applying an incorrect style.
I still really like the idea of constraint based systems, but I think they are more suited to (web)applications rather than documents.