I'm loyal to a fault when it comes to TV watching, I'm afraid. Unless it gets horrible, I tend to stick with a series until its end. This isn't the case for all shows, mind you, only the ones that were weekly watchers; there have been plenty of shows that I watch only an episode here or there, but in which I never become more invested.
"The Simpsons" is a show that I probably should give up on at this point because the entire last third of its run has been disappointing (not completely lacking in humor, but certainly not up to its first 13 seasons), but I literally have not missed a single episode and can't imagine not watching it now. Every week since I was 11, there it's been.
"South Park" is a show I gave up on once before and will again if this season doesn't improve. Turns out the last time I abandoned it was shortly before four of the show's strongest seasons. I probably watched Numbers and CSI too long. I quit watching CSI when Fishburne came on, but it was more background watching than anything else. I like to watch TV like that while reading a book.
"ER" was another show to which I was too loyal. Stuck with that one to the bitter end, despite a string of bad seasons. Ditto "Buffy the Vampire Slayer" and "Angel," though I've liked both shows' later seasons more in re-watching them than I did when they were new.
Other than "South Park," to which I eventually returned, the only show I can think of that I fully gave up on was "Homicide: Life on the Streets." The first few seasons were genius and are the reason that series is one of my all time favorites, but when that show started to suck it went from zero to unwatchable in an instant.