A few years, ago, I bought one of the last high-end rear-projection TVs based on CRTs - a Pioneer Elite 620HD. I did some basic calibration with the Avia disc and did some other minor adjustments, but never got around to getting a real calibration done.
Calibration is the process of getting the TV to be as close as possible to NTSC settings - the same settings that were used when the program was created. That means getting colors and gray levels as close as possible to what they should be.
But, if it's a high-end TV, why isn't it set from the factory to meet NTSC settings? Well, the simple fact is that TV manufacturers play games to make their TVs stand out in showroom settings. That generally means pictures that are far brighter than they should be (with corresponding poor black levels), colors are off, and the picture is over-sharpened.
Some newer TVs let you choose a setting that's close to NTSC, but in most cases, calibration can make a big difference. If you have a LDC or Plasma set, start with a disc like Avia and see what you get out of it (Avia also has calibration for surround sound, which may also be useful).
In my case, my set needed cleaning, focus adjustments (because it's a rear-projection set), convergence (because it has 3 CRTs in it, one for red, blue, and green), and geometry (because it uses CRTs). You can find local techs in my area who can do calibration, but because my set is fairly rare these days, I wanted an expert, and hooked with David Abrams from Avical, working out of LA, when he was on a trip in the Seattle area.
David is a really nice guy and did a wonderful job. He spent 90 minutes on the geometry of the set (making sure straight lines are straight in all 3 colors), and about 60 minutes on the convergence (aligning all three colors). I only have about 5% of the patience he has when he's doing those kinds of things.
So, after cleaning the set, setting the focus, setting the geometry and convergence, he was on to setting the gray levels and colors. With his test pattern generator (running through the TV's component inputs, which is all I use...) and his $18K color analyzer, that part went pretty quickly. He then worked through all my sources (Tivo HD, DVD, XBox 360) and verified that everything was set right. Finally, we looked at some source and he did some final tuning.
The results were pretty impressive.
The downside is that the differences between HD feeds is now really obvious - some look great, but others really fall short.