First 3 seasons were great. I thought season 4 and 5 did not quite meet those expectations. I seem to remember suspension of disbelief became more difficult at that point...OK it is 24 so a certain amount of suspension of disbelief is required but season 4 and beyond required a whole other level. I seem to remember 7 and 8 being far beyond the realm of comprehension as Jack appeared to become a Marvel superhero before our eyes.
I noticed people here seem to be forgetting seasons 9 (2014) and 10 (2017). Both were streamlined to 12 episodes which kept the storylines taut and did away with some unnecessary filler. Season 9 (which took place in England) was actually quite good and appeared to a return to form at least at the level of season 4-5 and possibly as good as the first 3 seasons. Season 10 introduced a new main character and the quality was on the level with the seasons 7 and 8 unfortunately. Weak writing! How many times can the lead go against the orders of CTU in 12 episodes!?
The show was fairly revolutionary for its time as there were few, if any, serialized evening dramas on tv at the time back in 2001, especially the main networks. Producers back then, thought it would be ratings suicide to have a drama that did not have self-contained episodes with a tidy beginning, middle and an end, within the allotted 1-hour time frame. After its first season, 24 consistently achieved ratings that would put in the top-20 - 40 shows on tv. 24 along with Lost proved the serialized concept could attract the eyeballs of the public on a weekly basis...and here we are, less than 20 years later and it appears serialized shows are the norm.
Shout out to David Palmer - one of my most favorite supporting characters on tv ever!