There are two types of message journaling for Exchange; message envelope journaling is the preferred method if you need to capture messages for compliance purposes, because it captures not only the message (the P2 information, i.e. the message contents), but also the message "envelope" (the P1 information, which contains specifics about who a message was sent to, including recipients who were blind copied, and expanded distribution list membership). Think about a letter you'd get in the mail - the envelope would be addressed to you, at your address, but the letter inside could say "Dear John, ...". Without envelope journaling, all you've got for reference is the "Dear John" portion of the message, but knowing who the message (i.e. letter) was actually addressed to (i.e. who received it) is also important contextual information.
If you're implementing a third-party archiving product, it will (or should - don't pick one that doesn't understand message envelope journaling) know how to handle messages that are journaled with the envelope data.
If you're not implementing a separate archiving product, and plan to use the journal mailbox as an archive, there will be some hoops to jump through to read the actual messages. However, if you're saving these messages for
compliance-related purposes, you want the envelope data, and I would strongly advise you that the journal mailbox is an inadequate long-term archive, especially for compliance-related purposes.