I put a Banks GitKit on my 2000 7.3. It was expensive, but I managed to save a ton on labor by installing much of it myself. I started with a turbo saver and that was a great convenience item. Next was a tranny controller and air brake. That was a definate upgrade that was well worth the money. Next I built the air flow part. Intake and exhaust upgrades, then the engine tuner. Next was the high performance turbo wheel, waste gate actuator and gauges for boost, egt's and trams temp. There were marked differences in every step.
The engine performed well before I started but these definitely "woke it up". The low end performance was way better. The engine was much, much more responsive to the throttle. Long hard grades like the gauntlet and Loveland passes was much easier with seemingly less effort from the engine. My TT weighed in @ 9000# and was 11' high.
I did have to replace the transmission at 110,000 miles but was told by almost anyone that that was about the lifespan of it anyway, so I don't think the kit had anything to do with it.
All in all, I think it was worth the cost. I gained some fuel economy although not nearly enough to cover the price of the parts, but that wasn't the goal to begin with. I did have huge performance gains, which was the goal.
Guess it just depends on what kind of results you are looking for and what you're starting with.