I've just finished installing my second rebuilt 4EAT in my 92 turbo (something like 315k miles!). The last transmission failed when a thrust bearing disintegrated - having nothing better to do, I did tear down the transmission to see what happened. Most of the transmission is fairly logical, at least if you've seen automatics before. The problem with doing a rebuild is just getting the sheer number of parts right - even my rebuilder commented that the control body got pretty interesting. Also, there are a fair number of measurements that have to be taken to ensure the proper thickness of shim plates, and probably more than one or two specialized tools necessary. If you're really adventurous and want to take a fun ride down the learning curve, go for it. If you want confidance in a pretty complex mechanism, go to the professional rebuilders. You'll still have lots of fun taking the beast in and out! Have fun!
By the way, if anyone is interested in a rebuilt turbo 4eat, I might think about replacing the damaged parts in my old unit....