My assessment of this time honored question is this:
I keep putting enough money into my LDS as a way to make sure that they continue to be open, and there when I need something right now! (small parts, etc.) If I see something online that is WAY cheaper than what they are offering, then I have to go that route. I have to do what's best for my pocket!
That said... I HAVE seen the odd item that they did have at a very competitive price, or cheaper...case in point, I wanted to get a new dive computer and checked the usual online outlets to see who had it cheaper.... I wandered over to the dive shop on a night that a friend of mine was working, and asked "So how much is that here?" It was actually SIGNIFICANTLY cheaper than any of the online places, so I picked it up right then and there...
Now, as far as air fills? I have a friend that formerly worked there, who bought his own compressor (Nitrox/Trimix capable as well) and I use him for fills because A. he's cheaper, (he charges on consumption rather than a flat fee) B. He's much more convenient and C. I think he takes much better care of his equipment than the LDS does. (granted they ARE within specs, but he goes the extra mile)
They know I have taken my business elsewhere, and they know why, when I do. It's a large industry, and a lot of people are trying to make money...I spread my dollars around!