I don't know, there are a couple of people that had a difficult time with a few of the Gym Leaders. I guess it all depends on what level range you've reached to them though. I hope they do put on some kind of challenge for me however, and I'm pretty sure since B/W has been leaning towards levels being high for various points in the game (like the Elite Four levels, Victory Road's, and even how long it takes for one Pokémon to evolve in levels) I'll have a chance to get the challenge I'm expecting. n_n;
As for looking through the designs of each Gym Leader, I would say I like Kamitsure the most and by far, my favorite Gym Leader for this generation. I love electric type Pokémon, and her design fit overall is quite outstanding. Her outfit fits the electric typing but as for the challenge she puts up I'm not quite sure. Of course she's only the fourth Gym Leader so she won't be too difficult to defeat and so there's not really much I can say to that. XD;