When stock firmware of any router gets derided as "crap" in the forums, stability is usually one of the items mentioned. But "stability" is often hard to define. However, there is a key instability cause that Merlin addresses for the RT-N66U and RT-AC66U.
ASUS enabled GRO (Generic Receive Offload) by default in early firmware versions and still gives users the option to enable it in current versions. Generic Receive Offload merges similiar network packets so that the OS and CPU see less of them, which reduces network overhead.
GRO is supposed to increase network performance. But in the Dark Knight, it has been shown to cause instability. User experience has shown that simply disabling GRO made the router more stable. So Merlin firmware completely disables GRO in the name of stability.
Top: Merlin firmware (no GRO option), Bottom: ASUS firmware
Merlin also contains a few bugfixes, such as fixing the crash on VPN/NAT Loopback access of LAN devices.
A big benefit of the Merlin firmware is that you keep the OEM look, feel and functionality. ASUS has a decent AiCloud, which allows you to access your home network via your mobile phone anywhere, and Merlin firmware retains this.
For support, since Merlin is built off of ASUS' firmware, many of the discussions in the official ASUS Wireless Forum are relevant to Merlin code. This is an advantage over loading up Tomato or DD-WRT, where OEM forums have little to no relevance.
But Merlin isn't just about bugfixing; Eric has added a few features, too. You have the option to shut off the router's LEDs, which is nice if you don't want it lit up at night or be a distraction in a media room.
A Wake-on-LAN page has a drop-down of all MAC addresses with any resolved names, which you see in the image below. This makes it easy to select a device to be woken up.
Merlin firmware WakeOnLan
(An alternative to waking your clients at the router may be the Fing mobile app that we reviewed with its cloud features back in August.)
On the Clients List, any unresolved clients will pop up a page from the IEEE's OUI (Organizational Unique Identifier) database if you click on the MAC address, i.e. OUI Lookup. This is nice when looking at connections on the router and wondering which clients they represent. While the lookup doesn't provide a device's actual name, the OUI Lookup will show the vendor that the MAC address belongs to.
Merlin firmware Unresolved clients OUI Lookup
Also added is the option to save Traffic Monitor statistics to USB vs. RAM only so they can be retained on a reboot. The image below shows the custom traffic monitor saving location and the option for disk spindown.