I'm on my third Asus RT router, and they've all run Merlin. One feature I particularly like is if you add a USB stick to one of the ports (I use the USB 2 port) you can get it to log usage to the stick.
The stock firmware records usage, but forgets the information if there is a reboot/power cut, so Merlin added the ability to store it to the USB and it reloads if it reboots. So you get an idea of usage over time.
Just one of many small tweaks that makes Merlin better in my view.
Some other people have replaced the whole firmware with equivalents of Advanced Tomato, but these often lose some of the enhanced WiFi features, so I wouldn't recommend unless you are not using the built in WiFi.
80/20 (2/jun/14) at 470m - Sync history highest: 64/9 (Sep/17), 54/6 (Jan/19), 51/6 (Mar/19)
20 years of broadband from 1999's ntl:cable modem trial - Live BQM