Jomic works great to convert cbr to cbz or pdf
archiver (rucksack) works great too, except it doesn't have a batch process that I could find and I had to rename the cbr files to rar to convert to zip. Jomic is a lot better and faster, especially if you already have a collection of comics you want to convert.
For people that care, cbz format seems to work better for comic reader apps such as Droid Comic Viewer on Android, less memory issues compared to cbr. I'm using my Nook Color to read comics and it works really well.