I've been a
www.tirerack.com guy for more then a few years now.
The trick is to have someone who can install them cheap. Otherwise the savings over buying locally isn't that noticeable.
Obviously if you show up at Town Fair Tire (for example) with 4 tires from tirerack.com they hose you for every single installation charge they can think of.
A local garage or a friend with a garage is the key.