I had an MRI done this summer and for me it was very useful. It was the only way for the docs to really see what was going on in my joint. They suspected something highly unusual but couldn't tell from the tomographs. The MRI confirmed it. We were also then able to send my MRI results to a TMJ conference with my doc and then off to specialists around the country to get other opinions (mostly to avoid surgery).
The MRI is not fun. It's really noisy and long, and you have to crank that plastic thing open in your mouth. But its WAY more fun than an arthrogram!! Good luck.