Having a trainer or following an instructor is a great thing in small doses. The problem with trainers, currently rearing its evil little head, is trainer dependency. We are all becoming too dependent on having someone telling us what to do and when (and how) when it comes to working out. So much so, that we have no idea how to possibly go about it on our own.
Sure, this is not a problem if you're a celebrity like Jessica Simpson and can afford to hire a trainer to spend every waking minute with you, but most of us cannot. The real key is finding a balance between the two. So use your trainer as a way to learn what you need to do (and how), but make sure that you are taking notes so you can also do it on your own, when the trainer is not there. Maybe sign up for only a few sessions at first so you don't only associate working out with your trainer.
The same rule should apply to group classes as well. I am all for switching it up for variety and taking classes to bust boredom, but make sure that if you're without the class, for example if the instructor stops teaching the class suddenly, you're not completely flummoxed if you have to workout on your own.
Fit's Tip: If you simply can't workout without someone leading you through it, then by all means go with what works, I am not trying to talk anyone out of exercising. Just try and figure out ways to do it on your own too.