Does anyone know a decent app to monitor bandwidth usage, ideally from the terminal

After some searching, this appears to be a known issue. OS X somehow isn't fully aware my Dell monitor, and is not applying the same font smoothing. The font smoothing can be set using:

defaults -currentHost write -globalDomain AppleFontSmoothing -int 2

See this superuser and macrumors threads.

Depending on what you want displayed there are several options available using Terminal:

• top: real-time sorted display of running processes such as memory and CPU usage
• iostat: I/O summary statistics for terminal, device and CPU operations
• vm_stat: Mach virtual memory statistics
• diskutil list: disk/volume capacity
• df: used/free space on mounted partitions
• fs_usage: real-time file activity for both disk and network
• nettop: updated information about the network (a bit like top for net I/O)
• w: who is logged in and what they are doing; plus a brief system load summary
• ifconfig and ipconfig: network interface and IP protocol details

Most of these commands have a huge list of options.  It's probably best to consult the man pages for details.

I usually rely on:

• top -u -s 10 to identify CPU-hogging processes;
• fs_usage -f filesys or fs_usage -f network to identify processes generating a lot of disk/network load.