What you get from a good lbs is attention to help with fitting you on a right bike. Big box stores normally are faceless. So the person manning the bike station on that day may be from the garden centre or electronics. Then when it comes to prepping the bike... yowzah!
However I notice some big box stores where I am are starting to carry some good quality bikes. So if you know what to look for, you may score a good bike for a good price. Just bring it to a LBS for servicing.