I've gotten all kinds of benefits from working with my trainer, but we focus on exercise, not weight loss. He did recommend Nancy Clark's book to me.

There is much variety in quality of personal trainers out there. I see some who don't pay attention to what their clients are doing and either don't know or don't see that their form is wrong while doing certain exercises. My trainer has a degree in exercise physiology in addition to personal experience with marathons and triathlons. (However I can still beat the pants off him in Words with Friends.)