Pacman downloads all packages before installing any of them. So if you have a bad Internet connection then you'll be stuck at the first (download) step. It will not leave your system in broken state.
That's true. However, I broke my system pretty badly with this update. I must have refreshed my package cache but not updated my system. Probably with an aborted `pacman -Syu`. Not realising the impact it would have, I later ran `pacman -S rtorrent`. This happily downloaded and installed the new ncurses library, but not any of the other updated packages. Anyway, this broke bash, zsh, xterm, gpgme, vim and more. I had to turn off the PGP signature validation in pacman (without vi or vim) and make sure not to close my working xterm and zsh sessions. It was a pretty tense half-hour working it all out and restoring my system. I got it sorted out in the end but I wouldn't be suprised if someone else experiences something similar.