My favorite feature by far is the ability to look at the git history of the methods themselves ... just the other day I had a bug that I was tracking down, and by looking at the git history I discovered that a critical change had been made to that method 1 year 12 days ago ... from that I was able to look at all of the changes to all of the packages that had been made in the same commit (the critical code had not only been moved to a different class but the critical method had been moved to a separate package ... and from that I was able to see that a bug had been introduced when not all of the critical method was moved ... bug fixed ...
Besides method versions, you can look at the git history for a class, package or metacello project .... these are thing that would take a bit of work to accomplish using just Monticello ... and if you were to attempt to do it, the simplest thing to do would be to copy all of the packages into a git repository and just use git:) Frank Shearar wrote some code that did something along these lines a couple of years ago ...
Dale