The only reason I could see wanting cards from one of these online companies is if you only want a small amount of cards as it is usually better for local print shops to run larger batches on the presses. (But even then you will probably be able to get 500 cards for something like 50 dollars on a minimum run)
However, you get your business cards, and when you have additional printing needs, returning to them gets you better results (especially if you paid on time). The printers I know (several of them, in fact) all cherish customers who pay on time and will bend over backwards for them.
It also becomes easier going back to them. They have material already, and can usually get what you need faster.
However, I still think it's worth the time and effort finding a good local print shop and developing a good relationship with them.