There's an interesting story over at ESPN.
Their "insiders" rank Garrett 30/32 in the NFL coaches. That's very low. They give a few reasons, but the bottom line is Garrett purposely put himself in a no-win situation in Dallas by working with the overreaching Jerry Jones. Then there are his late game gaffess which could have very well cost his team 2 playoff berths over his tenure.
Garrett's record is mediocre. It has not improved over the past 2 seasons. There are excuses, of course, injuries, especially on defense, but all teams suffer injuries.
Garrett had playcalling taken away from him last year, so that's not a sign of a strong head coach.
I don't think Garrett is that bad, but I don't think he's that good. Regardless, I don't think any head coach can really succeed in Dallas as long as they have an owner who plays GM instead of having a football guy with a football background as GM. So if Garrett is fired after this season, who would be willing to work for Jerry other than another yes-man who is willing to let the owner/GM dictate his coaches and draft picks?