Various people have commented on other threads that they didn't have great results from Performance Max campaigns. In our case we tried one which started out with massive impressions, low CPC and low click through rates but it didn't develop into anything useful so we deleted it.
Our Google Advisor had encouraged us down the PMax route, and our most recent advisor also recommends PMax campaigns, saying existing campaigns are old hat, we need new campaigns, PMax or Demand Generation campaigns which they believe are the future.
I don't really see a great difference between PMax and broad match traditional campaigns, which cost us for less than relevant keywords.
Anyhow, anyone got a good word to say about Performance Max campaigns on Google?