The Pressflow site mentions that it is used by many Drupal 6 sites, and in the FAQ, in answer to the question "What's the deal with Pressflow 7?" says
Pressflow 6 made significant improvements to Drupal core in scalability and performance and most of these features have been contributed back to Drupal 7 core. Until well understood performance patches are created for Drupal 7 Pressflow development will be slow. Additionally, as Drupal 8 matures, significant enhancements will likely be backported to Pressflow 7.
That sounds to me like a very indirect way of implying that there are lots of problems with Pressflow 7 that cause people to avoid using it, but stated in a way that no one would understand unless they're already aware of the problems. The answer seems like more of an explanation (indirect and implied) of why there are problems, than really an explanation of what the situation is with Pressflow 7.
So, can anyone tell me in plain English what the situation is? Is there some reason I wouldn't want to replace a normal Drupal 7 site with Pressflow 7? Or some reason it is simply impossible? (Or is the problem simply that Pressflow 7 has not been optimized to the same degree that Pressflow 6 has?)