Been studying this only for 2-3 weeks. Though a lot of the content of this exam seems to be what I did way back in 2010 when I did the ISCW exam for CCNP.
Hasn't felt like a massive step in difficulty. It also helps that I am now 2nd line Network Support and have been since October '12 and so getting daily exposure to most things on the curriculum.
Material - CCNA Security Cisco Press guide, CBT Nuggets, GNS3...
I did buy an ASA5505 but I really haven't done much with it. I've done more in GNS with a virtual ASA than anything else, but as I intend to do the CCNP:Security straight afterwards then I haven't wasted any money in buying the ASA. I fully intend to use it in production at home