Having Challenge Mode unlocked has made this the most fun Pokemon campaign in a long time. Gyms actually feel like boss fights, which they haven't felt to me since I was a kid with Red and a habit of making full party teams without over-leveling.
For the first Gym, being 2 to 3 levels under and having the same amount of Pokemon (3), I basically had to use a bunch of switching, growling, paralyzing, and such to win. The second gym (I had four pokemon this time) was even tougher until I gained a level (so 1 to 2 levels under, I think) so stuff evolved (namely Lucario and Flaaffy with Charge). It went from being way harder to slightly easier than the first. One rule I'm thinking of making for myself, after being able to pull it off twice, is reset upon any of my Pokemon being knocked out, to make up for the fact I'm using more Pokemon (in the second case). I've been paying attention to natures, because they do seem a little more necessary. The wild Pokemon selection being appealing from the start helps too.
I can think of plenty of ways Challenge Mode could be better (level caps), but it makes a big difference in keeping me engaged. Now I know I won't touch a Pokemon game without such a mode being enabled (and that's assuming they give me the means to enable it). X&Y couldn't be more unappealing to me right now, though if my sister suddenly decides girls are allowed to play Pokemon, I may check it out to keep her company.