Thought I would share my thoughts and pics of using a QuickJack 5000 on my 79 Trans Am.
Overall, I am pleased with the QuickJack, but I do still find myself using jack stands about half the time. This is mostly due to the fact that it takes longer to take out the QuickJack and position it under the frame rails than it does to just grab a floor jack and some stands. If I am going to be doing any kind of work that will have the car in the air for a long time, or if I need to use the jack stands for other parts of the project then the QuickJack comes out.
First, its not cheap. I paid about $1,500 but you can probably find deals on them from time to time.
As far as height, its great. Technically it will hold the car at a variety of heights, but really you should only use two because those have locking points. And its really stable - I give it some good shoves and it doesnt move. I dont really trust any kind of jack stands or the QuickJack entirely so I tend to throw a couple of tires under the car somewhere that will give me an escape route if a worst case scenario happens or at least not die.
The biggest drawbacks to the QuickJack are that it takes up space in the garage (albeit not much), its darn heavy (dont believe the picture of the guy on the box with one under each arm...), take some time to setup for each use, take some time to get used to using, and the biggest drawback is you cannot access the car from the sides - only the front or back. Technically you can position the QuickJack left-right and get under the sides of the car, but then you cannot slide under from the front or back.
The QuickJack is absolutely fantastic for doing oil changes, rear end work, suspension work, gas tank, really anything not directly under the car. It's not ideal for transmission work, exhaust work, or things like that.
I typically put one pad as far forward on the frame rail as I can get and the other just in front of the leaf spring mounts at the back of the car. The two stands need to be parallel to each other for proper function.
Let me know if you have questions.