Working with a trainer is a great idea for many people. A good trainer can give you the fundamental skills you will need to make the best out of every minute you spend in the gym. Developing good habits and techniques, while keeping you moving forward, are what trainers do on a daily basis. A good trainer is not a cheerleader, cheering you on as you run on the treadmill; instead they are a coach, giving you feedback and direction while constantly refining your skills.
But who is training your trainer? Is your trainer located in a "big box" gym where they are certified internally and the majority of the training is on how to sell more training, not make you stronger and move better? Or is your trainer working on their own or have their own gym? Are they certified by a recognized organization, not a mail order or Internet-based certifying body, and are they working with you outside their skill set on things that they don't have any experience with and you are their "guinea pig" until they get it right? This is a dangerous situation and one that is encountered way too often...
Do you want to know what the sign of a great trainer is? They are constantly working with their peers and their mentors to improve their skills. It's not about learning the newest pole dancing yoga craze, but spending time delving into the basics of the strength and conditioning game. Practice, under the watchful gaze of a more seasoned trainer, can improve anyone - even your trainer.
BTW - if you walk into a gym and the walls are plastered with certificates from their trainers attending certifications then you are at risk. Many trainers think that chasing paper makes them better trainers but all that it really proves is that they have never spent the time and work to get good at any one thing. Beware of the trainer with tons of credentials - they are looking for the shortcut to expertise and excellence.