Actually, I found never got quite as bad as they said, but it did go from fascinating to interesting to mediocre-but-like-popcorn. I think the_stalwart was on target when he said that volumes 5 through 11 felt like filler. There's about 4 to 6 volumes worth of story spread over 12 volumes.
I didn't really care for the ending. It wasn't the what, but the how. That is, in a series filled with convoluted mind games, the one that resolved the series did not feel substantially different from any other. There was no particular reason for that one to resolve it, except that the author decided that this is where the series would end.
In addition, the reader really can't figure out what's going on in advance. This is not a deal breaker in itself. The Sherlock Holmes mysteries, for example, cheat all the time (and not always on purpose). But, the Holmes stories are short, and the manga is long and padded. It's a lot more of a problem.
Again, it isn't the what that's a problem. I have no objection to Light getting his comeuppance. Near is absolutely on target when he says that Light is no more nor less than a serial killer. But the final sequence had me expecting Light to see his way out of things. I fully expected Mikami to have written everyone's name down in what Light knew full well was a fake notebook.