Using an F5 server as reverse proxy cache (instead of Varnish)
I have used F5 as a load balancer with Drupal without issue. We were using varnish between Drupal and the F5 as a reverse proxy. 

Pressflow and varnish have been tested well together, F5 is quite a high end bit of kit so I don't know if anyone else has tried it. From what I remember F5 is very flexible so you should be able to get it to work in the same way as varnish (same headers being sent etc).