It really depends on your other alternatives as to whether its worth it. Almost all of them are "rust buckets" even in areas where rust is not normally a problem. I have an 81 with 260K miles on it. I enjoy driving it, its reliable etc. but its looking ugly from the rust. It makes no sense to repaint because it just comes back out. I'm a bit embarrassed to still be driving it but its still a lot more fun than driving my new suburban to work every day.
It can be cheap to fix if you are willing to junk yard dive. The slightest accident totals them so there's lots of them in junkyards. The engine and drivertrain are solid as a brick.
The worste rust problem I have is where the spare tire sits in its well on the side. I was looking under the car the other day in the back and I was shocked to actually see the spare tire sticking out from the the bottom. Eventually, it would have just rusted thru and fallen out.
If you like to tinker go for it. There's a good Z car newsletter with lots of nice people willing to offer advice. Unfortunately, it will never look very good.
I'm kinda in the market for a later model max thinking it provides some of the same kick in the pants, but has newer features like new freon ac and air bags. I'm having a hard time parting with the old gal however. Have fun don