Actually they are worth of, but usually it is the common problem. The employer will not sent, because they will cost too much.
An individual cannot afford the amount to attend personally. An employeer will not allow because the person on whom they are investing money and sending to conference. How long stays with the company etc.,
But the conference are more worth, because the questions raised in the conference and answers are a lot worth than the amount spent on conference.
I have been lucky enough to have attended several conferences and feel that a large portion of their worth is in networking. You meet people who have tackled problems you are facing and you can use their experience to help you.
If you are looking to go to your first, I would recommend conferences with shorter presentations, so you do not get stuck in an all-day presentation that may not be applicable to your position.
I work in a large organisation that sends two test team personnel to the (Euro)STAR conference each year. I was lucky enough to attend and found it to be very beneficial.
I agree that if it is tructured into a number of short talks, you can pick and choose the best ones each day, and then in the evening you can network. Amazing as it sounds, a hotel with 800 testers in is a very interesting place to be.