I have a simple rule these days; if it doesn't feel right, I'm gone. In the past, Every time I should have walked but didn't, it was always a major disappointment. For the most part, walking in my experience relates to being duped in some way - it's either a different girl or it is the same girl but she bears no resemblance to her photos.
If you turned up to a garage to buy a car and found the car you wanted wasn't the one you went to see, you wouldn't buy it (unless of course it was a damn sight better than the one you were considering). Once you've walked, it generally becomes easier to walk thereafter