This show is truly excellent—at least, that’s true for the initial three seasons. The very first season stands out as particularly strong. The animation flows well, the characters are humorous, the opening sequence is quite good, and thankfully, there aren't any whipcrack sounds. Season two remains impressive, though not quite on par with its predecessor. A noticeable shift occurs starting with this season: production moved from Warner Bros.' internal studio to Cookie Jar in Canada, resulting in animation done in Flash. While Flash isn’t inherently a problem—other shows like Total Drama have used it effectively—it does sometimes lead to a rougher look, and certain characters don't translate as well. Dukey is an example I think. The core character personalities stay consistent; Hugh continues to be the best. Another issue that begins in season two is the repetition of plot elements. There are at least three Johnny X episodes within just this one season. Season two also brought in the whipcrack sound effect, but it’s not used excessively—maybe once or twice per episode. A new theme song was introduced as well, and it's essentially a recreation of Green Day’s “American Idiot.” Things really decline with season five. It’s genuinely disappointing. The opening credits are a letdown; they seem carelessly animated because the production studio had changed again, this time from Cookie Jar to Atomic Cartoons. Flash animation continues, but initially, there's no apparent difference. However, this is when the show truly falters, not only due to characters becoming exaggerated caricatures, but also because the whipcrack sound effect becomes far too frequent. In the 100th episode special, "Johnny’s 100th Episode," the episode holds the record for most whipcracks—nearly 200. I'd say this show is a prime example of what it means to lose quality over time.

Source on TMDB