I'm compiling some new information for users who are looking to flash DD-WRT onto their DIR-890L. The information contained in the first page of this guide remains generally relevant for making the first migration over from stock firmware to DD-WRT. You should also read every page in this thread.
Parts of the Peacock FAQ remain relevant but it is a little outdated. It is still a must read. If there is conflicting information always follow 890L-specific advice found in this thread instead. In particular you do not need to do 30-30-30 resets for the 890L, a factory reset on the 890L is simply using a needle to hold down the reset button for a period of time, maybe 5-15s, until the lights start flashing. For increased stability I would recommend making DHCP IP reservations (i.e. setting a static IP) for all your devices.
The DD-WRT settings are very complicated, especially for the wireless settings with a lot of adjustments that you can make, and in particular, the SSID passwords have to be set in a different tab. For specific settings do read through every page of this thread, other posts in the DD-WRT forums, as well as the DD-WRT wiki.
For users that have been forced to upgrade to a newer version of the stock D-Link firmware as a result of publicly disclosed security vulnerabilities - I've noticed that the newer stock firmware versions tend to have complications with intermittent disconnects with the 2.4 GHz wireless on certain network environments, particularly those with a lot of wifi devices. There are no such issues with DD-WRT.
For issues relating to the LEDs on the 890L after flashing DD-WRT, see this post. Personally what I did is Administration -> Commands, paste the following and "Save Startup":
You must downgrade to stock firmware version 1.09 before migrating over to DD-WRT.
As of today, I have flashed the latest DD-WRT version of r44340, and its the most stable build I've tried so far. For other versions that I've tried the worst thing that I've encountered is periodic router reboots. If you follow all the instructions correctly bricking is low risk. Always factory reset before and after updating your DD-WRT firmware.
IMPORTANT: DD-WRT is not for people looking to be spoon-fed and you are expected to spend many hours on research before asking any questions. If you have spent many days/hours on your research and you have not been able to find an answer to your specific issue, please open a new thread to ask your question. If you do not get a response feel free to PM me.