Up until around 3 years ago, I appealed every year. I viewed it as holding the CAD to account.
In almost every case, I was successful. A friend in real estate would provide comps. I'd meet w/ a CAD employee who'd bring up the data that they were using for comps. I'd share mine and they'd agree w/ mine. In one specific case, the CAD employee pushed a function key and the data was re-populated, resulting in a 15% reduction. Ok.
In 2019 (I believe), the informal appeal failed and I did the formal appeal. Everything described previously was accurate. 3 octogenarians and a CAD employee are with you in a room. You present your position, the CAD employee refutes it using every manner of poor reasoning. "Mr. Boilerhorn appeals every year. He has a nice yard. Blah." I cannot respond, "Of course I appeal every year. People who do not are not holding you accountable." I was able to get a 15% functional obsolescence on my home because of it relative to homes in neighborhood. One of the panel said, "I do not understand one bit of Mr. Boilerhorn's math, so I side with the CAD." My math was basic price/Sq.Ft. as a function of Sq.Ft. My home is larger than others in the community; the previous metric was fairly linear - except for my house.
A few years back, I called one of the "pros" for consulting. He said, "I can make more money focusing 30 minutes on appeals than you can pay me to consult. But since you are on the phone..." It was enlightening, but I did not really learn that much. He did say that he (and darn near all of the other "Pro" tax appealers) formerly worked for TCAD. They know everyone there and they know the process. I had already figured there was some sort of positive relationship. in 2012 (or so) I saw one of the Pro appeals guys show up with 4 file boxes on a dolly. He was greeted cheerfully, shake hands with a few people, etc. I found that interesting. Also, your file is one of thousands he'll handle. You'll likely do a more thorough job and you'll definitely care more about the outcome.