To me breaching the 150K word barrier is the time when I can actually start thinking that I am really going to finish a book before the book finishes me. There are certain points in a book's life that go something like this:
20K: there's still time to stop and start again. What I've been writing is crap.
50K: there's still time to stop and start again but the pain of bining a month or two's work is immense. What I've been writing is crap, but it's a lot of crap.
100K: I've written so much that I can't start again but I'm losing the will to live. Maybe I should go back to accountancy. I don't know how to resolve the plot. All my characters are anodyne. What I've been writing is crap and it's incoherent crap.
120K: maybe some of what I've written ain't too bad. I have to dump/rewrite a couple of characters, usually the ones I spent the most time researching. Now is the time for a major read thru and assessment. I can't think of an ending. I'm fucked.
150K: I have a big pile of crap which needs to be polished (d'oh?). I still can't think of a denouement. There's too much sex/violence/swearing/exposition/boring shit. What I've got is mileage, which is great if I was laying a road, but unfortunately I'm writing a book.
180K: This is when I stop. There should hopefully be within this heap of over-written poo a half-way decent story of 150K. I start editing. I edit for three months. Nobody talks to me. I don't talk to me. I go to parties and stand there mentally editing my book. I refuse to read books on the grounds that these cocky bastards have finished their edit and are just taunting me. I change the ending. I change the ending again.
150K: I abandon my book and instead send it off to my agent. Why should I suffer alone?