I think that Jimmy is not suppose to be on an equal footing with Lois and Clark. He is suppose to be this young, eager beaver, annoying little brother. Justin played that much better.
To me Michael Landis looked too old for how I pictured Jimmy. In season 1 where Michael played Jimmy he looked ridiculous running around being afraid of Perry. He looked old enough to be a college graduate and in the work force for a couple of years in other words he looked almost as old as Lois and Clark. From day 1 I found Michael Landis annoying as heck as Jimmy and terribly cast.
Justin seemed a much better fit for Jimmy but I did have a major problem with the wardrobe they picked for Justin's Jimmy it just didn't look like him even if you were going for him not dressing with his career in mind.
