Me too. I’ve had three RT-AC68Us for a number of years now - two run stock firmware (one serving DNS and one as an AP) and one runs Merlin (AP).
Unlike stock, the Merlin regularly stays up and stable for months at a time. I just checked the admin interface and last boot was 62 days ago - when we had a power outage.
Couldn’t be happier. There are lots of good reasons not to use consumer routers, but for me these have been an “if it ain’t broke…” situation.
Unfortunately it is not an option if you hope to increase the longevity of your ASUS router. Once ASUS upstream stops supporting your router, so does Marlin. As they did with my AC-66U - which makes sense because they rebase their changes on top of official ASUS firmware.