For me, it was absolutely worth it! I played around with the idea of getting a trainer for a few years, but always changed my mind because I thought I already knew so much about working out. Also, I didn't like the approach of a lot of trainers I watched at the gym. I simply do not care for the bootcamp-type yell-at-your-client style of training. And then there was the cost. I didn't want to pay for a "glorified spotter"
BUT -- when I switched gyms in November of 2002 and realized that I was in a real rut -- I decided to try a trainer for a change. I only signed up for a month of training (1 time per week) at first to get a feel for whether I wanted to continue and so I wouldn't have to shell out a bunch of dough. I had watched this trainer with other people and he apparently knew what he was doing (safety is my #1 concern) and he also has a BS in Exercise Physiology.
I noticed a lot of changes in my body almost immediately because he pushed me to work harder than I would on my own. I lifted a LOT heavier with all exercises (especially legs). Even though he didn't really teach me much about exercise or training -- the motivation was well worth the money. A nice side-benefit is that I met quite a few people at the gym through him. I used to be the type to go in, do my workout at lunchtime, and then leave without talking to a single person. But he kind-of brought a social side out of me and, although I'm not there to socialize, it is nice to meet new people and to feel a comaraderie with the lunch-time regulars.
He also inspired me to become a trainer myself. I had never considered fitness as a career until talking with him and seeing that a person can make a decent living as a trainer. I got my certification from the Cooper Institute in April and got re-certified by AFAA. I'm teaching classes now and am currently looking for clients to train (while also holding down my full-time engineering job).
So...for me, there were a LOT of benefits to getting a trainer. I think you should try it! If you don't like it after a couple of months...you can find a different trainer or go back to working out on your own.
Good luck!
Shonie