Honestly, I don't feel "regretted" watching any anime, no matter how bad or boring it can get.
(except when it's edited for content though, then the whole story and series is screwed up...*cough*YTV*cough*Viz's edited version of Naruto*cough* ahem!!) 
However, I do feel a little disappointed in watching certain types of anime like for example: I just finished watching the first volume of
Saiyuki Reload Gunlock today and it was pretty good and funny. But somehow, I feel the plot and the story a little confusing...as if this was continued from another series which I haven't seen the others before. So it got me all lost.
And that's when I feel this "disappointment" and wishing not seeing this anime series or other series alike that are continued in sequence of storylines and plots which I haven't seen them before.
Hmmm, it turns out that after the short research of that series is actually the 4th
(or 3rd?) installement to the entire story altogether of
Saiyuki. Hell, I didn't even have the chance to see the original
Saiyuki,
Saiyuki: Requiem (movie), and
Saiyuki: Reload yet!!
^.^ lmfao
can you see the face in the clouds? click