I really like that aspect of homebrew. I don't let anything else use /usr/local, and it means I can install things without using sudo, so no risk they are messing something up elsewhere in system directories.
But you risk the installation messing with your data and home directory. MacPorts uses sudo because it drops privileges to a specific 'macports' user for the build process and applies sandboxing. Therefore, the execution of Makefile rules and other build scripts cannot harm your data.
