I still use a Linksys WRT54G that has been stable and mostly solid. When Cisco purchased Linksys, firmware upgrades ceased and the model is listed as no longer supported. Previous firmware upgrades addressed some security issues.
There is a recent version of
DD-WRT for this particular router. For me the main reason to flash to DD-WRT would be security issues. Is there a place to look for security issues - general internet searches just say that routers are vulnerable and give no specifics.
The router sits behind a Cisco 678 modem and I've yet to implement a firewall.