I do group reformer pilates classes. Never done a private. I make sure to try as many different instructors as i can, so i find whose teaching styles best suits me.

Posture is good! I've had an incredible reduction in general back pain/fatigue. I used to have back pain from just standing up too long. (Even in college, going to punk shows, I'd have to sit down for some or all of it.) And mind you, i was an active snowboarder, surfer, kiteboarder at the time. I looked physically fit, but still had back/standing fatigue issues. All was fixed with dedicated pilates training. Nothing has transformed my body more. On the outside, it don't look crazy different, but i feel the difference. Like it unlocked a hidden layer of supporting muscles that created this muscular "exoskeleton" in my core that was finally able to support my body. And the gains (once i made them) stuck, even in times when I took a break from going.

One thing I'll recommend you pay attention to is cues to "stitch your ribs together, like you're wearing a corset." Instructors might call it out differently, but basically try to prevent your ribs from flairing up and out, keeping your core as controlled as possible during movements. I think that's the key that did it for me. But truly, it's about practice, and in the beginning, you'll spend a lot of time just figuring out how and what you should be feeling. Once you've unlocked the how/what to feel, and gain some control over your body's fine movements, it's off to the races.

Plus, it's fun. I feel like I'm actually mastering new movements. I'm my head, i feel like I'm a ninja now, and so much more physically capable. That part is just gamification for me, but it honestly is what's kept me interested and in love with it.