It's been 10 years ago, but a fellow golfer I knew went through a full GolfTec program. He went from a guy who rarely broke 100 to breaking 90 on the regular. Happened pretty quick, I want to say six months. So that was good, and impressive.
The bad was two parts. The first was he turned into a really slow golfer. Painfully methodical - a really long preshot routine which he would often restart when something went wrong, reading every putt from 8 angles, standing over his bag forever.
The second was his swing was mechanical - not a lot of power, no real "rhythm" to it. Almost like swinging the club was hard work. He shaved 15 strokes in a hurry, but it appeared he had taken that swing as far as it could go and I didn't see any way he could improve from where he was without big changes. A few of us were still quite a bit better than he was, and he got frustrated in a hurry. Within a year, nothing else had really changed and breaking 90 was difficult again. And he was still getting lessons.
He ended up joining a different course the following year and I never saw him again.