This week's book giveaway is in the Agile and other Processes forum. We're giving away four copies of The Mikado Method and have Ola Ellnestam and Daniel Brolund on-line! See this thread for details.
Of course you can use Flex with S2; you can use Flex with any back-end ramework. The REST plugin exposes services that can be consumed by a Flex application; if you're already using S2 it might be a good mix, if you're not, then there are many other options.
GWT is very useful if you have the same components on several of your jsp pages. I mean is you can reuse the components. and also GWT is good if you don't want the headaches of dealing with multiple browsers for javascript compatibility.