I usually bring my lunch, but I let myself go out about once a week for a treat. The key is to vary the lunch enough so that you don't get bored with it. I find leftovers to be a great way to do that.
I always bring lunch and eat at my desk and then go outside later. I find that if I don't bring my lunch, I end up feeling guilty about spending money on lunch when I know I could've brought it.
During the school year I usually buy lunch on campus because I'm too lazy to make it. This summer I'm working as a research assistant and I have way more time on my hands, so I've been packing.
At past jobs i used to buy ALL the time because I figured it comes to the same thing: buy food & make lunch or buy food & eat right away. It's not the same whatsoever. If you're looking to save money, definitely make lunches at home. Either cook extras at dinner & bring that or bring some frozen meals- there are some really good and healthy choices out there! But definitely treat yourself once in a while :)