The effect of an ending depends on the series. Cowboy Bebop gets away with having a relatively* unremarkable last couple of episodes because Bebop was never about the overall narrative.
On the other hand the Madoka Magica Rebellion movie misses an open goal by having an ending which, while intellectually interesting in post mortem, leaves most viewers feeling disappointed due to the lack of a clear resolution (this probably isn't helped by how effective the original series' ending was).
Ultimately there are a lot of factors involved. Episodic shows like Bebop don't tend be remembered for their beginning middle or end. Longer stories have to be more about the journey than the ending in order to work. Buffy the Vampire Slayer would still be my favourite TV show even if the ending was terrible (as it happened the last couple of episodes were actually decent), because I didn't watch through seven series for an ending. I watched them for the characters and writing.
Shorter non-episodic series have a harder time justifying a poor ending simply because the ending makes up more of the content. A poor ending will leave an aftertaste to the entire thing, while in a longer story this may be overshadowed by many other superior moments.
Case in point, I prefer Madoka Magica to FLCL simply because Madoka's ending is perfect while FLCL's marks perhaps the weakest point of the series**, in spite of the fact that in every other regard FLCL is probably better; I finished watching Madoka thinking "Wow, that was the best thing I've ever seen", while I finished watching FLCL thinking "err... the ending didn't quite follow on from the rest of it as one might have liked."
*This is Bebop we're talking about, the last two episodes are still excellent.
**Again, by no means an insult.
Why are introductions so hard?..... Mate......
I saw the toothbrush scene courtesy of Gigguk's review and I was like....
I imagine the owner might be doing some proper racing with this car as well?
. Well that being animated makes more sense now.
