I love my PT. I have plenty of drive and determination in the gym, but he has helped me progress and get some variety in my training. If I just trained by myself, I'd be constantly lifting heavy weights, and probably not using the correct technique. With him, I have learnt how to do power lifts, but also plyometrics, cable work, etc. He has taught me more about functional fitness rather than just brute strength.
It's a personal choice though- but I always think PTs are a good idea when you first start. Huge amounts of people I see lifting in the gym have poor technique, which prevents them from getting to where they could.