please advise why Varnish cache persists, header disappears

You asked for testing so I have tried this module on two sites. I am not ready to try Varnish 3 (I contributed to another thread on do on the VCL but never got it working and reverted to Varnish 2.1).

The module woks great. Small server, 2G RAM but optimized for Drupal.

I have not really understood why Varnish will not cache the whole page on a D7 site without this module; presumably it does with D7 Pressflow, which I have not tried?

The second puzzle is that I have set VCL to send an X-Cache HIT or MISS header. In one site, that header disappeared after enabling the module, but not on the other site not.

A third thing I do not understand, is that after using the module, disabling it, and restarting Varnish to clear the malloc cache, Varnish continues to be able to cache the page for some hours at least, (even though Varnish did not cache the page before the module was installed). This makes testing difficult.

I ran ab test on a simple page with 100000 requests and concurrency of 200. With default cache (whether or not connecting to Apache via Varnish) the requests timed out, as it is far too much for my small server. With APC as default cache the tests were completed in about 16 seconds on the second run. With this module providing default cache they were completed in about 4.7 seconds on the second run :-)