If CRX contains jackrabbit, then what does it add?
Is there an up to date comparison or feature list that shows what CRX buys me over jackrabbit and sling?
Also, any information that would help someone determine when and why they would choose crx vs jackrabbit/sling?
CRX provides the repository browser as well as a number of administrative features that are not provided with Jackrabbit.
Adobe has some documentation on the technical foundation of CQ that you may find helpful: