Screenshots

Asuswrt-Merlin being a mod based on the original Asuswrt firmware from Asus, it retains the same web interface users are already familiar with:

Main webui

(Click on any of the screenshots to get a large version)

The changes are fully integrated in the original web interface.  Here's the DHCP static reservation page, with the new Name field: DHCP
There is a new Tools menu on the left where a few additional pages were added, such as the Sysinfo page:

While Asus has now integrated the OpenVPN server and client support into the official firmware, the Asuswrt-Merlin implementation retains some additional flexibility, such as the ability to configure two separate server or client instances.

Since the settings reproduce most of those available on Tomato, it means that most configuration guides available that are designed for Tomato can also be applied to Asuswrt-Merlin.

A basic Wireless site survey page was added, to help determine the optimal channel you should use for your router.
Traffic monitoring is also enhanced, with the addition of traffic breakdown per device, either historical, or realtime.    
DNS-based filtering of specific clients, allowing you to secure your children's devices through services such as OpenDNS or Norton ConnectSafe, while retaining unfiltered name resolution for your other devices.  
Enhanced Wireless client list, with automated content refresh.  Clients connected to the Guest network will also be identified as such.
Policy-based routing for your clients while using an OpenVPN client tunnel on your router.  This allows you to select specific clients, or destinations, that will use your VPN tunnel, while everything else will keep directly accessing the Internet through your ISP.