What are the main differences (strengths and weaknesses) between checkpoint and other reproducibility packages like packrat?

Checkpoint is a more comprehensive package from a reproduction standpoint. This is because it works in tandem with a server side solution, the checkpoint-server. Most of the burden is on the checkpoint-server, not the end user. Packrat does not have a concept of client or server, and a user could still find themselves in "dependency hell". Packrat requires a user to rsync, scp, or some how get your Packrat file and code to another user. That can be a cumbersome and tedious process.With checkpoint package, all you need to reference in your code is the snapshot date, and then you can commit your R code to a public or private repository, such as github.com. Then all another user has to do is pull your R code down, run it, and they will be using the same packages as you, because they are using the same snapshot date.In short, the checkpoint-server brings order to the world of R packages. It knows exactly how things looked at a given point in time, and everyone sees the same stuff, therefore everyone is on the same playing field.

For more information please review this page:
http://projects.revolutionanalytics.com/documents/rrt/rrtpkgs/