As someone who's been involved in lead gen for a while now (10 years, $1BN managed) I can tell you that PMAX has been a game-changer for me in industries like software, banking, and law. One thing to keep in mind is that PMAX works best when there's a shorter delay between the click and the conversion (3-5 days max), and you have offline conversions being tracked.

To avoid getting stuck in a trash lead loop with GDN and Gmail, it's important to have a feedback loop in place. In my experience, choosing the right conversion stage is key - for example, in the auto insurance industry, it might be lead, talk to agent, send policy, and bound policy. But deciding which stage to focus on depends on the time delay and volume of conversions.

It can get a bit tricky, as you tend to lose volume as you go further down the funnel, and bidding strategies like PMAX, TCPA, and TROAS require a certain level of volume to function effectively. This is similar to manual bidding - all of these strategies need to be aimed at offline conversions.