It might feel tempting at first to put something like this in
block log all
- just to make sure you don't miss anything.
The PF user guide contains a detailed description of how to make PF log to a human readable text format via syslog, and this does sound rather attractive. I went through the procedure described there when I set up my first PF configuration at work, and the experience sums up rather neatly: Logging is useful, but by all means, be selective. After a little more than an hour the PF text log file had grown to more than a gigabyte, on a machine with less than ten gigabytes of disk space total.
The explanation is simply that even in a rather unexciting Internet backwater, at the far end of an unexceptional ADSL line there's still an incredible amount of uncontrolled Windows traffic such as file sharing and various types of searches trying to get to you. The Windows boxes on the inside probably weren't totally quiet either. At any rate: put some sensible limit on what you log, or make arrangements for sufficient disk space, somewhere.
When you enable logging of network traffic, your system will start accumulating potentially sensitive data about your users and the network resources they access. Storing traffic data may in turn trigger legal obligations. The specific requirements (if any) will vary according to local legislation, make sure you check what requirements apply to you.