Whether or not you install it is up to you, but there is a lot more information about DD-WRT on their Wiki as opposed to their website: http://www.dd-wrt.com/wiki/index.php/What_is_DD-WRT%3F
Generally you do not want to update firmware unless you really need to, because it carries the potential to "brick" the device (render it useless). To give you an example, I have a Linksys WRT54G, and I am running DD-WRT because I had issues with the stock firmware, and it's a really old router. The router was freezing with larger downloads, and sometimes network devices would randomly lose their wireless connections.
Another reason to upgrade is you can unlock additional features, but the software itself and capabilities all depend on the make, model, and hardware version of your router. The features are more advanced and pretty specific IMO, but you can run VPNs, fine-tune wireless settings, and access stuff otherwise restricted or unavailable in the stock firmware.
If you are not having issues with your Internet connection I do not think you need to upgrade, but if you do decide to upgrade your firmware, please be sure to read any and all instructions available and follow them exactly. There are certain circumstances where if you make a mistake in upgrading the firmware, you will be unable to use or repair your router, so it's better to play with an extra one first!
Please let me know if I can provide any additional information.