I think one key to the success in North Africa was the change of commander at a very crucial time. Often the change of commander brings a new fighting style which can take the opponent, who is used to how his enemy fights, by surprise. The things Monty did well that can be attributed to him was the bringing of better morale and that he finally bothered to lay mines and prepare proper defenses unlike his predecessor.

As for Market Garden, with today's knowledge it seems like a very foolish operation but the intelligence was very bad, and the knowledge about how effective paratroopers could be wasn't really that well known by then after they had only been used a few times previously. Obviously also a lot of other commanders around him thought it was a good idea at the time. I'd say the guy is overrated, yes, but he wasn't totally useless either.