hmmm, well my 15" mbp seems to be pretty ok as far as i can tell, although if i was really picky i could say that the left side of my screen was also a little darker, but only from about an inch from the left side, after that it is very uniform! Is this the same as yours?
Also, people (myself included) have a tendancy to really scrutinise their tech and find faults that most people just wouldn't see. In the end, it comes down to whether the fault that you find would hinder your productivity or stop you from using your laptop as you originally intended to! So if a perfectly even display is that important, then take it back by all means, however, if it is something that you don't regard as having that big of an impact on you then keep it!