Last time I had a personal trainer, I hired her for several weeks at a time, 3x per week. The deal was that she'd get me started on a good weight routine, teach me how to do it all, and then I'd call her back after a couple months to get a re-vamp and update on the routine. She taught me how to use weights, what to do when and so on. She got me started on what weight to use for a particular exercise, what exercises to on what days - she made me a really nice chart and plan to follow for the next several weeks.
I was a total newbie to this, and she was fantastic - encouraging and helpful. She was almost more like a coach than a personal trainer. Just amazing, She was a 40-something Mom who was only about five feet tall, and yet - she played hockey every night with all kinds of athletes. Super strong, tough and in amazing shape, and yet - totally non-intimidating. A normal (albeit very strong) woman driving around in a mini-van. I liked her a lot!
If you can find someone good like this, then I'd highly recommend it. For personal training, I'd recommend meeting with the trainer 3x per week for awhile (maybe up to two months), until you get a routine going, then doing it on your own for a little while, and then bringing trainer back to tweak/improve the routine.
My trainer also gave me very good advice on what do to during non weight training days - I was in the gym five days a week, three with the trainer, and two on my own. I felt coached, encouraged and supported. Definitely worth it. I'd meet her at the gym at 6am on Mondays, Wednesdays and Fridays. She also provided very good instruction for Tuesdays/Thursdays when we didn't meet up at the gym.
I don't know if this is the norm, but I recommend two months on (with trainer), two months off (doing everything on your own), then two months back 'on' with a newly re-vamped program, and then another two months 'off' doing it on your own.