I'd say that by and large the show has had a pretty steady declining level of quality as the stakes were upped, and the show took itself less and less seriously. But that last season was a pretty sharp spike. Sort of like looking at Al Gore's CO2 chart where things wiggle and squiggle up and down a bit and then in the last century or so it spikes up.
Because that 8th season of 24 was just PAINFUL, I've not even re-watched it on DVD yet, but I will as I go through the series on DVD this next week or so.
And while the show's quality may have '"declined" over the seasons I'd say only slightly (other than season 8 and *maybe* 6) even at it's worst (aside from season 8) the show was still pretty darn fun and entertaining to watch. And, hell, there's even a *touch* of something to enjoy in 8. It's still just very, very, painful.
To Aldo I'd say watch through the whole series, even season 8. It's still a fun and enjoyable show all around it just never seemed to reattain the "spark" those first 2 or 3 seasons had.