Any third-party firmware, like DD-WRT, SlugOS, HyperWRT, are really for advanced users only.
Third party firmwares are for people who want to adventure with VPN Servers, VLAN, XLink Kai, IPv6, overclocking the CPU (of the router), boosting radio performance, adding additional security to a large access point, anti-virus scanning of traffic, endless port forwarding, WDS, AP mode and other options. I have a Linksys WRT54GL with DD-WRT combined with a bridge and a 24dbi parabolic grid antenna for maximum directional wifi distance and added security.
I would strongly advise you to study up on it heavily before you even make an attempt to flash your router with DD-WRT. Read every tutorial, FAQ and document you can. And make damn sure your specific router is compatible. If you fail to flash your router properly, you will "Brick" the router, rendering it completely useless.
Most of the time your stock firmware will be just fine for most users. Like Tigger said, 99% of the people wont benefit from third-party firmware.