I've used personal trainers...
...on and off (about 50 / 50) for the last 6 years....starting after I divorced wife #1 and moved to New York, so I figured I better get my ass in shpe for being single (again).
Most of the stuff here is good advice...but I will give you some more thoughts / feedback.
Decide in advance what it is you want to accomplish, with SMART goals (Specific, Measurable, Agreed - With Your Trainer, Realistic, Time Based).
With those goals in hand, interview a few different trainers - different trainers will have different backgrounds, strenghts and weaknesses. Do not assume the guy (gal?) with the biggest muscles is the best trainer - the best trainer I EVER had was a guy who actually was a tiny bit soft (he admitted that he didn't watch his diet super close). He was much stronger than the average person - he had to be in order to do his day job - but not a muscle head, and in fact not as strong as me once we reached my goals.
What he DID have was experience...years of training...and knowledge through the constant research and classwork he did. He was a dancer (modern ballet), which meant he had to be stong enough to effortlessly lift his female partners, and it also required he be very flexible. He also posessed UNBELIEVABLE "Core" strength...in combination with amazing balance and flexiabilty. When he was done with me I no longer had back pain (it has since returned), I had lost 50+ lbs, and I could do simply amazing things that took balance and "core" strength...like...kneel on a swiss ball while doing 60lbs (each hand) shoulder presses...kneel on a swiss ball and curl 50lbs dumbells...or stand on a swiss ball (yes...stand on a ball...it can be done).
Make sure anyone you work with is CERTIFIED. And it isn't a bad idea to make sure that either they (or the gym) are insured.
I have found that the compatibility / relationship between yourself and your trainer is very, very key. Me - I need someone who will push me, but who understands that when I say "no more" I MEAN IT. This is because I already push myself pretty hard (old habits - football - die hard). So I dont' need someone in my eargiving me shit once I say "enough". On the other hand, some people need a trainer because they have no idea how to push themselves...they think when it gets a bit difficult, you are supposed to stop. My last two trainers have been PERFECT....but the guy I had in New York was a pain in the ass. I need knowledge and motivation...I don't need a cheerleader / drill sargent. You have to decide for yourself what you are looking for...which is a good reason to work with multiple trainers until you find one that is a fit.
Cost - depends on the gym and the level of experience of the trainer. I've paid as little as $50/hr, and as much as $90/hr. You can pay more...especially at boutique gyms in some cities (like New York), but I refused. My current guy is a tad pricy...especially after i go see him 3 or 4 times in a week...but I am paying for him and the gym, because he has his own studio / gym. it is worth the extra 10 - 15 an hour to not have to deal with crowds or noise or wating to use equipment. Again, decide what works best for you.
Summary:
1) Know what you want to accomplish in advance so you can discuss it with each trainer you interview (yes - INTErVIEW THEM...because you want to know how much experience / knowledge they have, and you want to check the personality fit)
2) Verify that anyone you work with is certified.
3) Work out with a few trainers to get a feel for how the relationship witll work ongoing, then pick the best fit for you
4) Expect to pay at least $50/hr...and more for someone who is more experienced.
HTH...