Like many have said on here, they are absolutely worth it, if you're willing to put in the work. Just like everything in life, you will only get out of it what you can put into it.
There are no "magic bullets" that going to an instructor will give you. They will give you prudent information and techniques that, with a great deal of hard work, will improve your game tremendously.
Just like Fatboy said in a thread he started, pool is rough right now because it is too damn hard, everyone wants instant gratification. I have been to the same instructor many times (yesterday in fact), and I believe it has drastically reduced my learning curve. But I still play 2-5 hours a day trying to incorporate what I have learned.
In summation, if you think they can tell you something that will immediately make you a shortstop, don't go. If you are willing to listen, change, and work your ass off to acheive what they tell you, absolutely it is the best thing you can do for your game.
-Brandon