I've been using it for a while, got a Linksys WRT160N, DD-WRT works on a fair few routers actually, look at the website for info, mines a wireless N.
The firmware basically lets you do pretty much anything by unlocking all the settings available to you. I actually didn't realise how much there was nor do I yet know what half of them do. What was useful though, is that whereas before my router was having trouble with signal reaching the ends of the house, DD-WRT let me jack up the transmitting power, full coverage now. It also lets you do things such as Bridge routers (use one router to connect to another router and act as a big, wireless hub) among other things, if you feel comfortable flashing your compatible router, it's definately worth a shot and, even if you have no use for it yet, I reckon you might find something worth it just by browsing.
Btw, on a less personal, more business note, it has features to enable you to create a public access point complete with splash screen login for people connecting like you'd get at a commercial hotspot.
There are alternatives such as 'Tomato' but I went with DD-WRT because it was compatible.