1) I've never had an ongoing trainer, however back when I did my Dip.Fitness, I spent a lot of time doing mock-sessions with my peers, both being the trainer and being trained. Back when I was at SSG, I began my membership with a coaching session with Nick.
2a) Liked: About mock sessions -- being put through workouts and protocols I wouldn't have thought of otherwise. About Nick's session -- he wasn't trying to impress me by killing me or doing strange things; instead he focused on helping me to nail my key lifts and offering programming recommendations to support progress on those lifts/reinforce technique points.
2b) Disliked: About mock sessions -- they were often over-zealous in the surprise factor/not knowing what's coming next; they didn't always hit the mark on what I was aiming for or what I needed on that day (eg one time I had done a full body strength session the day before, so, given the choice of strength or endurance, I asked to be put through an endurance workout, forgetting that my definition of endurance was "high reps" while the trainer's definition was "all the reps"). About Nick's session -- there's often a chasm between being a strength guy and providing high quality customer service; objectively the session was spot on, but subjectively the human element could be worked on. It wasn't bad, per se, just not all it could have been.
3) Mock sessions: Didn't get much of a say in the matter, and probably wouldn't have known what to look for if I did have much agency over who trained me on any given day. Nick's session: I wasn't actively looking for a trainer, but I had been looking (in general) for a place that facilitated powerlifting and weightlifting. I took a leap of faith and, when Nick recommended the coaching session while I was signing up for my membership, trusted that his background would make him a helpful coach.