This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori 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.