I've only tried the Total Gym once just so I could make fun of my friend for having it in his home. From what I've seen it looks the "advanced" exercises you perform on the total gym are isolation exercises performed at a high incline.
Lets just assume for analysis that the Total Gym is completely rock solid: the cables could hold the weight of a car, the handles are Japaneses steel, the pad can hold 3 tons, and all that.
If that's the case, you'll still plateau quickly by doing presses, hack-squats, and rowing movements with only fractions of your own bodyweight.
Then you see on the infomercials big gym rats who are astounded at how hard the total gym is. But do you see what they have these guys dong? All the 'advanced' and challenging exercises performed on the total gym are isolation exercises like chest flys, rear delt rows, bicep curls, tricep kickbacks, pullovers, and sissy squats.
I think if a commercial quality total gym were to be made it could easily take the place of a lot of the free-motion brand machines. If someone were to design a total gym with a weight bar that could hold as much added weight as a hack-squat and then devised a way to keep you on the platform the total gym could replace all cable machines. But that will never happen.
But most importantly, don't buy a Total Gym. Everyone wants to know your bench max. No one cares what you can do on the total gym.