There are those who will say that there are only two places that serve a good cheesesteak, and they're both in Philadelphia. I've never been to Pat's or Geno's (yeah, I watch a lot of Food Network), so I can't argue that directly. I can, however say that I've had a few very good non-Philly cheesesteaks in my time. Admittedly, there have also been a few not-so-good ones, but they're not worth mentioning.
There aren't a lot of cheesesteak places in Pittsburgh, probably because of some corss-state rivalry thing (Pittsburgh has Primanti Bros.). Sure, there are restaurants that happen to serve a cheesesteak, but I think we can agree that it's just not the same thing. Recently, however, I found a good cheesesteak place right in the middle of downtown Pittsburgh.
The place is called Cory's Subs, and they do indeed make a damn good cheesesteak. It's a pretty new restaurant, too. So new, in fact, that Google Maps Street View still shows the place that was there before. Unfortunately, they don't have a website for me to link to but this link will do a Google search for you, so if they do get a website up...well, you get the idea.
I've eaten at Cory's twice (It's clear across town from where I work, or I'd go more), and both times, I enjoyed a fantastic cheesesteak with peppers and onions (they offer mushrooms as well, but I'm not a mushroom guy). The staff is friendly, the food is good...if it was closer to where I work, I'd be in trouble.
I'm not going to try to compare it to Pat's or Geno's (Never had them, remember?), but I will say that if you find yourself in Pittsburgh and craving a cheesesteak, this is where you gotta go.