I've only had 2 good trainers out of about 6 that I have used in my lifetime. Some trainers were lazy, rude, money hungry, or just completely crazy (I have experienced all of those types). My last trainer was so awful (he was all of those things that I just mentioned) that I ended up quitting the gym because of him. He made it very uncomfortable for me to continue working out there. Trainers turn over pretty quick because they don't earn that much when they are employed by a gym - they only get about 40-50% of what you pay the gym for a personal training session. If you find a good one, I would recommend that you ask the trainer if they will workout with you outside of the club at another facitlity because they do tend to move on quickly for better pay (I can't blame them at all). I did that with one of my good trainers when he quit my gym. If I need a PT session with him, he will workout with me independently at my current gym. Be aware that some gyms don't allow outside trainers because they want you to use their trainers. Just pretend that they are one of your workout buddies. Keep the fact that he is your trainer on the QT, or you may find yourselves being evicted. At the current place where I workout, no one would notice or even care if I brought my trainer in because it is at a university gym. Just don't have the guy come in with "Personal Trainer" emblazoned across his shirt.
The 2 good trainers that I had were passionate about their work and motivated me to succeed. I don't use a trainer anymore, but with what I learned from those 2 trainers, I'm confident that I can have the same success on my own. Just remember that you control the purse strings. If you're not happy with a trainer, you have every right to find another one. I would also recommend against purchasing too many sessions in advance. Many gyms will try to talk you in to buying 20-30 sessions in advance, and will even throw a discount at you to get you to do it. I did that once and I later regretted it. When I got stuck with that awful trainer, I had to use up the sessions before I could quit the gym, or I would lose all of the money that I paid for the remaining sessions. When I tried to switch to a different trainer, he went completely ape shit on me. I can tell you from personal experience that it's very uncomfortable having to workout with someone that you don't particularily like just because you have sessions to use up. Another thing to be aware of is that gyms can expire your sessions if you don't use them up after one year (it's usually well hidden in the contract fine print). They try to talk you into buying more PT sessions knowing full well that most people give up on working out after a couple of months. The unused sessions that expire after one year are pure profit for the gym as you don't have the right to a refund. Again, check the fine print in the contract - it will probably state that the PT sessions are non-refundable.