Just a quick/late update - I have now been to all three of these places. Competitive Camera is a very well-stocked place for digital, pro/commercial etc. with knowledgeable staff but no longer carries film and has no dedicated used analog equipment section, at least when I was there. But if I was a working pro in this day and age, I'd definitely do business with them.
Don's is a cool place in its own way but basically just a mess (literally - disorganized piles!) of old used stuff - cameras, lenses, darkroom, and motion picture cams/projectors but they do sell the black & white films I use for a reasonable price. There is VERY little new camera equipment but oddly they had the full Zeiss lens line in stock for both Canon and Nikon - including the latest models compatible with the digital SLRs & prices were pretty much what you'd pay at one of the big internet places. "Todd" there was very helpful and let me take out several and handle them. They have a fair amount of b&w darkroom chemicals & paper from Kodak & Ilford - they actually still have boxes of Kodak b&w paper on the shelf which I know hasn't been made in a while. I checked to see if it was Ektalure (loved that stuff!) and when I saw it wasn't, I didn't pay more attention to what it was...maybe Polycontrast?
Arlington Camera - by far my favorite of all three, but a longer drive so I don't get there as much. Yes, they have all the latest/greatest digital stuff but their film and darkroom chemicals/paper sections are more extensive than I've seen in a small independent shop in quite some time. I went in at about 11:00 on a weekday and the place was bustling - nice to see. They had plenty of salespeople who seem to know their stuff and have a great customer service ethic.
My two cents...