Don't let yourself think those 300 pages were wasted just because the landing sucked. The thing is, a bad ending can definitely sour the whole experience, but it doesn't erase what came before. If you were genuinely engaged for most of it, you got something out of that time - the characters mattered to you, the story pulled you in. A rushed or pointless ending is frustrating specifically because you cared, which means the book did its job for the majority of the read.
That said, yeah, endings matter a lot to me. I've definitely put down books feeling cheated when an author clearly just ran out of steam or didn't know how to wrap things up. But I've also re-read stuff where I initially hated the ending because I realized it made more sense the second time around, or because I'd built up unrealistic expectations. The real test is whether you'd ever think about that book again - if you find yourself wondering what happened to those characters or talking about it with someone, the ending didn't completely kill it.
Where I draw the line is when the ending actively contradicts what came before or betrays the characters. A rushed finish is annoying, but a pointless one that also feels out of place? That stings more. Either way, it's worth asking yourself what specifically bothered you about it. Was it unsatisfying emotionally, or does it just feel lazy? Because those are different problems, and one of them might matter less to you on reflection than it does right now.