Security of Airport Extreme Base Station

I've been looking at upgrading my years old router with something more modern. After having tried several newer models from various companies, I think I am ready to enter the world of Apple branded routers. There have been a number of issues with dropped connections, inexplicable lockups, and outdated firmware with the Linksys, Netgear, and D-Link hardware I've had experience using, and I've been told that the AEBS tends to be a nice, stable device. However, I do have some questions before I make a purchase. How is the security on the AEBS? Should I be concerned about the lack of an SPI firewall? Every router I've used previously has had this feature, so I am a bit uneasy about not having it with the AEBS.

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What about external network intrusion? Is SPI a major deterrent? I'm just curious to know how Internet-facing network security is impacted by the lack of SPI.