The fact that they are transparent about this, that we can opt-out if we want and that they're open source makes me feel more than comfortable sharing anonymous usage information with them. Homebrew has been amazing for me!
Is an alert displayed to the user the first time it tries to send info to Google? If not, that is far from "transparent". It's good that they tell you how to opt out on the website, but I can't remember the last time I went to the website. Probably when I first installed Homebrew on this Mac.
% brew update
==> Homebrew has enabled anonymous aggregate user behaviour analytics