Definitely "turns to shit" lose all hope, you will be a lot happier with the series if you just skip it entirely
Honestly I am not sure what kind of anime bugs me more.
Good anime that turns to crud... or bad anime that could have just as easily been really good (well ok it has a few good seasons and two I really like).
I mean Vanguard for example is not a good anime, but it occasionally tries to be good and it surprises me For example:
The hero actually loses the world tournament and in fact the show actually does have a few genuinely good twists and character moments. Like suddenly the show goes "Are we a bad cash in anime or a good anime?... I cannot remember lets do something good". Sure it does lead to a REALLY boring series of episodes, that sort of explains why heroes don't lose major tournaments (because it just slows down the anime unfathomably...), but at least they attempted it.
As well the show had a genuinely awesome card battle where the female member of their team, who loses the majority of her battles, actually manages to pull a performance that would make Yu-gi-oh surprised. She actually cycles through her entire deck using card abilities and using her eidetic/photographic memory she basically knows exactly what cards she will draw.
Don't watch it though, as much as I compliment it... it is also an anime that just doesn't care about its own story. It is about selling cards and it will go back on ANY characterization if it means cards will or will not be sold. (For example to push new decks the characters will often use new ones... even though they wouldn't or doesn't match their style)
While I can also go to Keichi which is a ok anime but never becomes good.
Then there is Kenshin which goes TOTALLY insane last arc. Dear goodness... It is like watching Gi Joe and all of a sudden Harry Potter is involved like he always existed.
I'll have to contemplate it more...
I do know I have often watched animes ONLY because I found their premise to be interesting and continued watching anyway in some vain hope to see if somehow come to that.
For example Dog Days which is an anime about a fantasy world where war and true violence is so abhorred that they replaced it entirely with sports and competition. That just drawn me in so fast! It turned out however that it is a sort of wish fulfillment anime where the world is just some place you can step into and be the hero (which becomes more blatant in the second season and is where I lost interest).
I mean just imagine the story telling perspective. Can you challenge a kingdom to take them over? Can you force them into economic upheaval? The actual answers in the anime is a bit weak, battles in that anime are entirely optional and you can refuse them... As well you aren't even allowed to challenge other kingdoms to do overly harmful bargains. As well most of these battles are not done at a bet, the money earned is entirely paid for by merchants for advertising.
Ohh and the Princess is a Popstar.
I keep getting bad animes but I don't want to stop looking for the good ones that explore these awesome concepts.