Full read/write of cartridge ROM, all known save chips. Yes, even Dezaemon 3D's triple SRAM is supported.
Special thanks to sanni for helping out with errata related to the lesser-known Flash revisions.
Dumping speed is about 2MB/sec.
Acey has been using his to dump Ninty proto carts (the tall ones that require 12v to work). I think the only other way to dump those carts with that power requirement was to use a Gameshark which was about 10-20 times slower and buggier.
Firmware 2.03 or later is required to use.
Since the PI/SI interface is exposed in the commandline USB Loader, you could use it as a starting point to do things like talking to the N64 capture cassette or modem over USB (Why, I have no idea, but it should be possible).
Also, the commandline tool lets you dump any PI range which means you can dump 64dd IPLs as well as initial registers. In theory, you could even reflash Gamesharks with it.