First off, when you buy it you can use it *as is*.
Plug it in and it works, reliability, end of
If you want to go down the open-source route have a read of
http://en.wikipedia.org/wiki/DD-WRT
or
http://wiki.openwrt.org/OpenWrtDocs
Its time well spent, IMO
Even if you decide not to, the linksys WRT is regarded as one of the best routers in the home environment out there, even in its stock state.