Disclaimer! I don't know much about lenses.
However, I do have some common sense. If you think you need "talking out" of it, understand that even though it may be a pretty sweet deal, your money will be MUCH better spent elsewhere. Things more suited to your needs and wants will come around and 550 is a solid chunk into any purchase.
Save your money for something better, man. I work in a ski shop surrounded by nice gear I could get for cheap but every time I'm tempted to look up the price on a cool hoodie or jacket or ski I have to stop myself and look at the bigger picture. That money will feel SO much better going toward something you NEED.