For me, final shipping cost is usually less than the 7% local tax I'd be forced to pay at my local hobby store. Add to that the less expensive purchase price, and in many cases ordering over the 'net is much much cheaper.
I have nothing against mom & pops, and I rather like the convenience. I like walking in and seeing what's new. I am willing to pay a premium for the convenience should I need something the hobby store can provide that a 'net retailer can't (immediate purchase capability and "see it myself" selection). For small purchases and "have to have it this weekend" items, I regularly go into my local shop to buy. It just makes sense for me if I am making a large purchase in bulk to go with a 'net store.
One beef I do have with 'net stores, at least in the hobby sector, is the less-than-capable web store software they use. There are some out there that are passable, but I haven't found many that approach the features of Amazon or Buy.com with the extra "help" you receive (detailed product descriptions, wish lists, lots of pics). Towerhobbies.com is nice, but mostly for R/C. Without a helpful interface, you still feel you are dealing with small-timers, which concerns me from the standpoint of customer service and support (I hate it when they don't have phone numbers or addresses listed on their website - what are they trying to hide!). There ARE inexpensive alternatives for small store webfronts, but mostly it's just a reluctance from the owners. Shame - they could reclaim lost brick-and-mortar traffic with a 'net storefront that makes the customer feel at home.
Sorry, I do customer service and retailing for a living. I guess I'm biased ;-)
- Snacko