There was a small documentary a few years ago on how he works, and he's absolutely a workaholic and dedicated to commentating. He loves his job and loves football. He's also a color commentator, so of course he's going to be a little more animated at times. It's similar to the hate Joe Buck gets - Joe Buck is consistently ranked as the highest or near the highest play-by-play guy with the least amount of mistakes every season. He's great at his job. Both of them are.
There have been some absolutely terrible commentators in the past - anyone remember Dennis Miller when he tried out commentating for a second? How about Jason Witten?