It's About Job Satisfaction
The point of your job search is not salary negotiation; it's finding a job you'll be happy with, that you'll grow with, and that will allow you to be yourself. If your starting salary isn't the one you dreamed about but the job presents the right opportunity, think about how much easier it'll be once you've had a chance to make yourself invaluable to the organization.
On the other hand, if the salary or benefits fall far short of your realistic expectations, despite all your efforts to negotiate, nothing says you have to take the job. Don't make the mistake of accepting a position with which you're fundamentally unhappy. Trust your instincts — if you're dissatisfied with the employer before your start date, don't bet the situation will get better.
Generally, a high salary doesn't buy job satisfaction. It may seem like the person with the BMW parked in the employee lot has the best job, but she may actually be unhappy at work. If you truly feel like a job will give you a lot of satisfaction, and you will be able to live comfortably on the salary being offered even if it isn't as high a salary as you would like, you should consider accepting the offer.