I like DDWRT... I never got into it 100% though (only like 80%) only because even with this software on these routers, they still can't do what I need them to do... VPN concentrators, VLAN's, VLAN trunking, etc...
A VPN concentrator probably needs a fair amount more processing power (especially for the encryption/decryption) than the 200-250 MHz you'll get out of a WRT54G/GS. I suppose the 5-port limitation is also a show-stopper for VLAN use. How awesome it would be if the internals were more expandable... and x86-based too.
I have two WRT54GS units myself (HW version 2.2 I believe), with DD-WRT on both of them. I once used them as regular 11g WAPs all the time, then one of them became really unreliable... it would stop processing network traffic after about 24-48 hours of uptime. I stopped using it for a year or two. Now, having come back to it, the thing is perfectly reliable. Or, I might just be using the other one. :/ It's hard when they're identical.
With two, it's so handy to have one always set up in WAP/router mode at home, and the other set up as a routed bridge. I've been bringing the bridge around on my little technical outings, along with a 8-port or 16-port Ethernet switch, in case I quickly need to give wireless access to several computers in the same physical area.