It seems like back darts are a pretty polarizing thing - I know just as many guys who love them as guys who would never even consider a shirt with that particular detail. A darted shirt, for those unaware, features two stitched down folds of fabric on the back of a shirt giving it a fitted appearance. The argument here is more or less focused on how slim you prefer your shirts. Lots of brands are selling off the rack shirts that feature a predominantly slimmer fit, but often these shirts can be fairly expensive or not slim enough depending on your body type. In most cases slim fit sport shirts come in standard small, medium or large sizing so it's not uncommon to size up for your neck or arm measurement, thus negating the slimmer fitting body. In this case it might make sense to take the shirt to your tailor or seamstress and have it darted. It's a fairly simple and cheap process costing around 15 or 20 bucks. For those of us on an even tighter budget the process of darting a shirt can help take an inexpensive shirt that fits traditionally (i.e. poorly for some people) and make it look much "better" - some sartorialists believe a darted shirt can visually take off 15 pounds or so. I'm kinda split on the idea of back darts myself. I have a few in my wardrobe and all of them happen to be broadcloth, spread collar dress shirts. In my eyes, back darts work better with specific types of shirts. I don't think I would ever have any of my OCBD's darted, but when it comes to broadcloth numbers (sans box pleats of course) I am not opposed to it whatsoever. It's a much more European feature and often looks best with shirts that fall into that stylistic category to begin with. If you really love the idea of back darts you can always buy shirts off the rack that come pre-darted. More upscale brands like Michael Bastian and Ralph Lauren Black Label dart a lot of their shirts so a trip to your tailor is unnecessary. Like everything else in this life it's a purely personal call that some people feel much more strongly about than others. Just remember, if I can see your nips, your shirt is probably too tight.
[Pictures courtesy of The Sartorialist.]